CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($)		Total	Common Stock	Additional Paid-in Capital	Retained Earnings		Accumulated Other Comprehensive Income (Loss)		Reclassifications Related to Funded Status of Pension	Reclassifications Related to Unrealized Gains on Available for Sale Securities	Treasury Stock
Beginning balance at Dec. 31, 2015		$ 792,237,000	$ 90,326,000	$ 210,545,000	$ 544,228,000		$ (16,457,000)				$ (36,405,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		71,392,000			71,392,000
Other comprehensive income (loss), net of tax		2,673,000					2,673,000
Cash dividends declared		(26,784,000)			(26,784,000)
Treasury stock issued		(115,000)			(2,945,000)						2,830,000
Recognition of restricted stock compensation expense		2,544,000		2,544,000
Tax benefit from stock-based compensation		9,000		9,000
Ending balance at Dec. 31, 2016		841,956,000	90,326,000	213,098,000	585,891,000		(13,784,000)				(33,575,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		72,968,000			72,968,000
Other comprehensive income (loss), net of tax		(4,643,000)					(4,643,000)
Cash dividends declared		(28,569,000)			(28,569,000)
Treasury stock issued		(689,000)			(2,183,000)						1,494,000
Recognition of restricted stock compensation expense		3,008,000		3,008,000
Ending balance at Dec. 31, 2017		884,031,000	90,326,000	216,106,000	628,107,000		(18,427,000)				(32,081,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of net unrealized gains on equity securities | ASU No. 2016-01	[1]				862,000		(862,000)
Reclassification of net unrealized gains on equity securities | ASU No. 2018-02		0
Net Income		105,334,000			105,334,000
Other comprehensive income (loss), net of tax		(391,000)					(391,000)
Reclassification of tax effects from the Tax Act | ASU No. 2018-02					3,427,000	[2]	(3,427,000)	[2]	$ (3,660,000)	$ 233,000
Repurchase S&T Bank Warrant		(7,652,000)		(7,652,000)
Cash dividends declared		(34,539,000)			(34,539,000)
Treasury stock repurchased		(12,256,000)									(12,256,000)
Treasury stock issued		(657,000)			(1,372,000)						715,000
Recognition of restricted stock compensation expense		1,891,000		1,891,000
Ending balance at Dec. 31, 2018		$ 935,761,000	$ 90,326,000	$ 210,345,000	$ 701,819,000		$ (23,107,000)				$ (43,622,000)

[1] Reclassification due to the adoption of ASU No. 2016-01, related to changes in fair value for equity securities reclassified out of accumulated other comprehensive income

[2] Reclassification of tax effects due to the adoption of ASU No. 2018-02, relating to $(3,660) relates to funded status of pension and $233 relates to net unrealized gains on available-for-sale securities.