Segment Information - Schedule of Segments Pre and After Tax Income, Significant Expenses and Reconciliations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue from External Customer [Line Items]
Total segment revenues	$ 1,360,110	$ 1,244,306	$ 3,972,041	$ 3,605,270
(Favorable)/unfavorable prior year casualty reserve development			70,096	182,748
Current year casualty loss costs			2,319,158	2,212,750
Net	819,031	765,658	2,389,254	2,395,498
Income (loss) before federal income tax	145,711	116,487	393,502	138,955
Total federal income tax expense (benefit)	(30,371)	(24,209)	(82,323)	(27,478)
Net income (loss)	115,340	92,278	311,179	111,477
Interest expense	(13,253)	(7,250)	(36,082)	(21,633)
Corporate expenses	(4,446)	(4,662)	(30,100)	(29,314)
Preferred stock dividends	(2,300)	(2,300)	(6,900)	(6,900)
Net income (loss) available to common stockholders, basic	113,040	89,978	304,279	104,577
Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development			90,000	211,000
Total Insurance Operations
Revenue from External Customer [Line Items]
Net	819,031	765,658	2,389,254	2,395,498
Dividends to policyholders	876	1,350	3,010	5,658
Standard Commercial Lines
Revenue from External Customer [Line Items]
Total segment revenues	955,424	883,568	2,816,049	2,583,519
Net	657,958	591,564	1,906,390	1,895,351
Dividends to policyholders	876	1,350	3,010	5,658
Standard Personal Lines
Revenue from External Customer [Line Items]
Total segment revenues	102,053	108,213	309,233	319,715
Net	89,290	106,113	235,936	291,897
E&S Lines
Revenue from External Customer [Line Items]
Total segment revenues	155,921	129,377	446,987	362,735
Net	71,783	67,981	246,928	208,250
Operating Segments
Revenue from External Customer [Line Items]
Total segment revenues	1,360,110	1,244,306	3,972,041	3,605,270
Net catastrophe losses	24,899	148,804	148,188	294,579
Non-catastrophe property loss and loss expense	169,630	146,747	521,565	503,407
Net	819,031	765,658	2,389,254	2,395,498
Commissions to distribution partners	212,527	198,915	636,454	577,467
Salaries and employee benefits	96,406	90,368	287,392	262,094
Other segment expenses	67,860	59,616	196,247	174,651
Total net underwriting expenses incurred	376,793	348,899	1,120,093	1,014,212
Dividends to policyholders	876	1,350	3,010	5,658
Income (loss) before federal income tax	163,410	128,399	459,684	189,902
Total federal income tax expense (benefit)	(33,887)	(26,614)	(95,503)	(38,655)
Net income (loss)	129,523	101,785	364,181	151,247
Operating Segments \| Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development	40,000	0	90,000	211,000
Current year casualty loss costs	584,502	470,107	1,629,501	1,386,512
Operating Segments \| Total Insurance Operations
Revenue from External Customer [Line Items]
Total segment revenues	1,213,398	1,121,158	3,572,269	3,265,969
Net catastrophe losses	24,899	148,804	148,188	294,579
Non-catastrophe property loss and loss expense	169,630	146,747	521,565	503,407
Net	819,031	765,658	2,389,254	2,395,498
Commissions to distribution partners	212,527	198,915	636,454	577,467
Salaries and employee benefits	96,406	90,368	287,392	262,094
Other segment expenses	67,860	59,616	196,247	174,651
Total net underwriting expenses incurred	376,793	348,899	1,120,093	1,014,212
Dividends to policyholders	876	1,350	3,010	5,658
Income (loss) before federal income tax	16,698	5,251	59,912	(149,399)
Total federal income tax expense (benefit)	(3,507)	(1,103)	(12,582)	31,374
Net income (loss)	13,191	4,148	47,330	(118,025)
Operating Segments \| Total Insurance Operations \| Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development	40,000	0	90,000	211,000
Current year casualty loss costs	584,502	470,107	1,629,501	1,386,512
Operating Segments \| Standard Commercial Lines
Revenue from External Customer [Line Items]
Total segment revenues	955,424	883,568	2,816,049	2,583,519
Net catastrophe losses	15,013	100,429	85,705	189,781
Non-catastrophe property loss and loss expense	118,749	95,864	379,423	335,410
Net	657,958	591,564	1,906,390	1,895,351
Commissions to distribution partners	173,294	162,947	516,871	474,267
Salaries and employee benefits	79,150	74,279	237,604	215,667
Other segment expenses	54,322	46,109	155,576	135,757
Total net underwriting expenses incurred	306,766	283,335	910,051	825,691
Dividends to policyholders	876	1,350	3,010	5,658
Income (loss) before federal income tax	(10,176)	7,319	(3,402)	(143,181)
Operating Segments \| Standard Commercial Lines \| Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development	35,000	0	80,000	211,000
Current year casualty loss costs	489,196	395,271	1,361,262	1,159,160
Operating Segments \| Standard Personal Lines
Revenue from External Customer [Line Items]
Total segment revenues	102,053	108,213	309,233	319,715
Net catastrophe losses	12,160	41,688	33,864	78,929
Non-catastrophe property loss and loss expense	39,907	37,964	104,667	125,192
Net	89,290	106,113	235,936	291,897
Commissions to distribution partners	4,083	7,498	17,958	23,121
Salaries and employee benefits	9,620	9,291	27,045	27,330
Other segment expenses	9,303	9,085	27,950	25,777
Total net underwriting expenses incurred	23,006	25,874	72,953	76,228
Dividends to policyholders	0	0	0	0
Income (loss) before federal income tax	(10,243)	(23,774)	344	(48,410)
Operating Segments \| Standard Personal Lines \| Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development	5,000	0	10,000	0
Current year casualty loss costs	32,223	26,461	87,405	87,776
Operating Segments \| E&S Lines
Revenue from External Customer [Line Items]
Total segment revenues	155,921	129,377	446,987	362,735
Net catastrophe losses	(2,274)	6,687	28,619	25,869
Non-catastrophe property loss and loss expense	10,974	12,919	37,475	42,805
Net	71,783	67,981	246,928	208,250
Commissions to distribution partners	35,150	28,470	101,625	80,079
Salaries and employee benefits	7,636	6,798	22,743	19,097
Other segment expenses	4,235	4,422	12,721	13,117
Total net underwriting expenses incurred	47,021	39,690	137,089	112,293
Dividends to policyholders	0	0	0	0
Income (loss) before federal income tax	37,117	21,706	62,970	42,192
Operating Segments \| E&S Lines \| Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development	0	0	0	0
Current year casualty loss costs	63,083	48,375	180,834	139,576
Operating Segments \| Investments
Revenue from External Customer [Line Items]
Total segment revenues	146,712	123,148	399,772	339,301
Net catastrophe losses	0	0	0	0
Non-catastrophe property loss and loss expense	0	0	0	0
Net	0	0	0	0
Commissions to distribution partners	0	0	0	0
Salaries and employee benefits	0	0	0	0
Other segment expenses	0	0	0	0
Total net underwriting expenses incurred	0	0	0	0
Dividends to policyholders	0		0	0
Income (loss) before federal income tax	146,712	123,148	399,772	339,301
Total federal income tax expense (benefit)	(30,380)	(25,511)	(82,921)	(70,029)
Net income (loss)	116,332	97,637	316,851	269,272
Operating Segments \| Investments \| Casualty lines
Revenue from External Customer [Line Items]
(Favorable)/unfavorable prior year casualty reserve development	0	0	0	0
Current year casualty loss costs	0	0	0	0
Other
Revenue from External Customer [Line Items]
Total federal income tax expense (benefit)	$ 3,516	$ 2,405	$ 13,180	$ 11,177

SELECTIVE INSURANCE GROUP INC 10-Q