CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning Balance at Dec. 31, 2020	$ 1,154,711	$ 103,623	$ 400,668	$ 710,061	$ 8,971	$ (68,612)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	87,866			87,866
Other comprehensive income (loss), net of tax	(10,000)				(10,000)
Cash dividends declared	(32,954)			(32,954)
Treasury stock issued for restricted stock awards	0			(3,988)		3,988
Forfeitures of restricted stock awards	(302)			1,389		(1,691)
Repurchase of S&T Stock	0
Recognition of restricted stock compensation expense	2,360		2,360
Ending Balance at Sep. 30, 2021	1,201,681	103,623	403,028	762,374	(1,029)	(66,315)
Beginning Balance at Jun. 30, 2021	1,188,733	103,623	402,053	746,472	3,605	(67,020)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	27,598			27,598
Other comprehensive income (loss), net of tax	(4,634)				(4,634)
Cash dividends declared	(10,991)			(10,991)
Treasury stock issued for restricted stock awards	0			(811)		811
Forfeitures of restricted stock awards	0			106		(106)
Repurchase of S&T Stock	0
Recognition of restricted stock compensation expense	975		975
Ending Balance at Sep. 30, 2021	1,201,681	103,623	403,028	762,374	(1,029)	(66,315)
Beginning Balance at Dec. 31, 2021	1,206,454	103,623	403,095	773,659	(7,090)	(66,833)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	95,250			95,250
Other comprehensive income (loss), net of tax	(107,877)				(107,877)
Cash dividends declared	(34,922)			(34,922)
Treasury stock issued for restricted stock awards	0			(135)		135
Forfeitures of restricted stock awards	(498)			1,832		(2,330)
Repurchase of S&T Stock	(7,637)					(7,637)
Recognition of restricted stock compensation expense	2,411		2,411
Ending Balance at Sep. 30, 2022	1,153,181	103,623	405,506	835,684	(114,967)	(76,665)
Beginning Balance at Jun. 30, 2022	1,178,358	103,623	404,841	809,644	(67,171)	(72,579)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,249			37,249
Other comprehensive income (loss), net of tax	(47,796)				(47,796)
Cash dividends declared	(11,747)			(11,747)
Forfeitures of restricted stock awards	(64)			538		(602)
Repurchase of S&T Stock	(3,484)					(3,484)
Recognition of restricted stock compensation expense	665		665
Ending Balance at Sep. 30, 2022	$ 1,153,181	$ 103,623	$ 405,506	$ 835,684	$ (114,967)	$ (76,665)