CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss)/Income	Treasury Stock
Beginning Balance at Dec. 31, 2018	$ 935,761	$ 90,326	$ 210,345	$ 701,819	$ (23,107)	$ (43,622)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,928			22,928
Other comprehensive income (loss), net of tax	6,176				6,176
Cash dividends declared	(9,317)			(9,317)
Treasury stock issued for restricted stock awards	(876)			481		(1,357)
Repurchase of S&T Stock	(12,287)					(12,287)
Recognition of restricted stock compensation expense	604		604
Ending Balance at Mar. 31, 2019	943,156	90,326	210,949	716,078	(16,931)	(57,266)
Beginning Balance at Dec. 31, 2019	1,191,998	103,623	399,944	761,083	(11,670)	(60,982)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	13,231			13,231
Other comprehensive income (loss), net of tax	17,342				17,342
Cash dividends declared	(11,051)			(11,051)
Treasury stock issued for restricted stock awards	(563)			53		(616)
Repurchase of S&T Stock	(12,559)					(12,559)
Recognition of restricted stock compensation expense	443		443
Ending Balance at Mar. 31, 2020	$ 1,176,251	$ 103,623	$ 400,387	$ 740,726	$ 5,672	$ (74,157)