Fair Value Measurements - Quantitative Disclosures of Fair Value Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
AFS fixed income securities	$ 9,275,424	$ 8,127,334
Equity securities	380,105	213,601
Short-term investments	587,911	509,318
Fair Value Measured at Net Asset Value Per Share
Equity securities	275,500	169,500
U.S. government and government agencies
AFS fixed income securities	143,398	120,155
Foreign government
AFS fixed income securities	9,985	9,302
Obligations of states and political subdivisions
AFS fixed income securities	560,960	451,230
Corporate securities
AFS fixed income securities	3,359,262	3,068,180
CLO and other ABS
AFS fixed income securities	2,478,741	2,033,149
RMBS
AFS fixed income securities	1,998,933	1,692,358
CMBS
AFS fixed income securities	724,145	752,960
Fair Value, Recurring
AFS fixed income securities	9,275,424	8,127,334
Equity securities	380,105	213,601
Short-term investments	587,911	509,318
Total assets measured at fair value	10,243,440	8,850,253
Fair Value, Recurring \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	38,836	35,518
Equity securities	104,567	43,279
Short-term investments	570,513	474,225
Total assets measured at fair value	713,916	553,022
Fair Value, Recurring \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	8,409,835	7,473,377
Equity securities	0	0
Short-term investments	17,398	35,093
Total assets measured at fair value	8,427,233	7,508,470
Fair Value, Recurring \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	826,753	618,439
Equity securities	0	808
Short-term investments	0	0
Total assets measured at fair value	826,753	619,247
Fair Value, Recurring \| U.S. government and government agencies
AFS fixed income securities	143,398	120,155
Fair Value, Recurring \| U.S. government and government agencies \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	38,836	35,518
Fair Value, Recurring \| U.S. government and government agencies \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	104,562	84,637
Fair Value, Recurring \| U.S. government and government agencies \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	0	0
Fair Value, Recurring \| Foreign government
AFS fixed income securities	9,985	9,302
Fair Value, Recurring \| Foreign government \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	0	0
Fair Value, Recurring \| Foreign government \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	9,985	9,302
Fair Value, Recurring \| Foreign government \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	0	0
Fair Value, Recurring \| Obligations of states and political subdivisions
AFS fixed income securities	560,960	451,230
Fair Value, Recurring \| Obligations of states and political subdivisions \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	0	0
Fair Value, Recurring \| Obligations of states and political subdivisions \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	553,330	443,804
Fair Value, Recurring \| Obligations of states and political subdivisions \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	7,630	7,426
Fair Value, Recurring \| Corporate securities
AFS fixed income securities	3,359,262	3,068,180
Fair Value, Recurring \| Corporate securities \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	0	0
Fair Value, Recurring \| Corporate securities \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	3,041,995	2,825,501
Fair Value, Recurring \| Corporate securities \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	317,267	242,679
Fair Value, Recurring \| CLO and other ABS
AFS fixed income securities	2,478,741	2,033,149
Fair Value, Recurring \| CLO and other ABS \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	0	0
Fair Value, Recurring \| CLO and other ABS \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	1,982,705	1,665,155
Fair Value, Recurring \| CLO and other ABS \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	496,036	367,994
Fair Value, Recurring \| RMBS
AFS fixed income securities	1,998,933	1,692,358
Fair Value, Recurring \| RMBS \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	0	0
Fair Value, Recurring \| RMBS \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	1,993,450	1,692,358
Fair Value, Recurring \| RMBS \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	5,483	0
Fair Value, Recurring \| CMBS
AFS fixed income securities	724,145	752,960
Fair Value, Recurring \| CMBS \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
AFS fixed income securities	0	0
Fair Value, Recurring \| CMBS \| Significant Other Observable Inputs (Level 2)
AFS fixed income securities	723,808	752,620
Fair Value, Recurring \| CMBS \| Significant Unobservable Inputs (Level 3)
AFS fixed income securities	337	340
Fair Value, Recurring \| Common Stock
Equity securities	378,259	211,767
Fair Value, Recurring \| Common Stock \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
Equity securities	102,721	41,445
Fair Value, Recurring \| Common Stock \| Significant Other Observable Inputs (Level 2)
Equity securities	0	0
Fair Value, Recurring \| Common Stock \| Significant Unobservable Inputs (Level 3)
Equity securities	0	808
Fair Value, Recurring \| Preferred stock
Equity securities	1,846	1,834
Fair Value, Recurring \| Preferred stock \| Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1)
Equity securities	1,846	1,834
Fair Value, Recurring \| Preferred stock \| Significant Other Observable Inputs (Level 2)
Equity securities	0	0
Fair Value, Recurring \| Preferred stock \| Significant Unobservable Inputs (Level 3)
Equity securities	$ 0	$ 0

SELECTIVE INSURANCE GROUP INC 10-Q