Securities - Fair Value and Age of Gross Unrealized Losses of Debt Securities (Details) $ in Thousands	Sep. 30, 2025 USD ($) security	Dec. 31, 2024 USD ($) security
Number of Securities
Less Than 12 Months \| security	4	41
12 Months or More \| security	93	96
Total \| security	97	137
Fair Value
Less Than 12 Months	$ 34,434	$ 366,890
12 Months or More	523,405	536,761
Total	557,839	903,651
Unrealized Losses
Less Than 12 Months	(251)	(5,989)
12 Months or More	(45,803)	(66,740)
Total	$ (46,054)	$ (72,729)
U.S. Treasury securities
Number of Securities
Less Than 12 Months \| security	1	5
12 Months or More \| security	7	5
Total \| security	8	10
Fair Value
Less Than 12 Months	$ 10,004	$ 45,045
12 Months or More	69,154	47,723
Total	79,158	92,768
Unrealized Losses
Less Than 12 Months	(2)	(362)
12 Months or More	(2,383)	(3,915)
Total	$ (2,385)	$ (4,277)
Obligations of U.S. government corporations and agencies
Number of Securities
Less Than 12 Months \| security		0
12 Months or More \| security		2
Total \| security		2
Fair Value
Less Than 12 Months		$ 0
12 Months or More		15,071
Total		15,071
Unrealized Losses
Less Than 12 Months		0
12 Months or More		(189)
Total		$ (189)
Collateralized mortgage obligations of U.S. government corporations and agencies
Number of Securities
Less Than 12 Months \| security	3	22
12 Months or More \| security	58	56
Total \| security	61	78
Fair Value
Less Than 12 Months	$ 24,430	$ 209,511
12 Months or More	308,439	318,104
Total	332,869	527,615
Unrealized Losses
Less Than 12 Months	(249)	(3,393)
12 Months or More	(32,916)	(44,885)
Total	$ (33,165)	$ (48,278)
Residential mortgage-backed securities of U.S. government corporations and agencies
Number of Securities
Less Than 12 Months \| security	0	1
12 Months or More \| security	18	21
Total \| security	18	22
Fair Value
Less Than 12 Months	$ 0	$ 8
12 Months or More	31,688	33,030
Total	31,688	33,038
Unrealized Losses
Less Than 12 Months	0	0
12 Months or More	(5,101)	(6,905)
Total	$ (5,101)	$ (6,905)
Commercial mortgage-backed securities of U.S. government corporations and agencies
Number of Securities
Less Than 12 Months \| security	0	9
12 Months or More \| security	10	12
Total \| security	10	21
Fair Value
Less Than 12 Months	$ 0	$ 88,040
12 Months or More	114,124	122,833
Total	114,124	210,873
Unrealized Losses
Less Than 12 Months	0	(1,741)
12 Months or More	(5,403)	(10,846)
Total	$ (5,403)	$ (12,587)
Obligations of states and political subdivisions
Number of Securities
Less Than 12 Months \| security		4
12 Months or More \| security		0
Total \| security		4
Fair Value
Less Than 12 Months		$ 24,286
12 Months or More		0
Total		24,286
Unrealized Losses
Less Than 12 Months		(493)
12 Months or More		0
Total		$ (493)

S&T BANCORP INC 10-Q