Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss)/Income	Treasury Stock
Beginning Balance at Dec. 31, 2015	$ 792,237	$ 90,326	$ 210,545	$ 544,228	$ (16,457)	$ (36,405)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	53,733			53,733
Other comprehensive income (loss), net of tax	10,814				10,814
Cash dividends declared ($0.60 and $0.57) (in dollars per share)	(19,824)			(19,824)
Treasury stock issued for restricted awards (90,115 shares, net of 23,946 forfeitures, and 110,643 shares, net of 5,717 forfeitures)	(115)			(2,945)		2,830
Recognition of restricted stock compensation expense	1,862		1,862
Ending Balance at Sep. 30, 2016	838,707	90,326	212,407	575,192	(5,643)	(33,575)
Beginning Balance at Dec. 31, 2016	841,956	90,326	213,098	585,891	(13,784)	(33,575)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	63,674			63,674
Other comprehensive income (loss), net of tax	1,180				1,180
Cash dividends declared ($0.60 and $0.57) (in dollars per share)	(20,899)			(20,899)
Treasury stock issued for restricted awards (90,115 shares, net of 23,946 forfeitures, and 110,643 shares, net of 5,717 forfeitures)	(688)			(2,383)		1,695
Recognition of restricted stock compensation expense	2,353		2,353
Ending Balance at Sep. 30, 2017	$ 887,576	$ 90,326	$ 215,451	$ 626,283	$ (12,604)	$ (31,880)