CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss)/Income [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2013	$ 41,213	$ 396,528	$ 481,732	$ (26,546)	$ (17,115)	$ 875,812
Balance (in shares) at Dec. 31, 2013	40,431,318
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			91,353			91,353
Other comprehensive income (loss), net of tax				57,266		57,266
Dividends declared:
Common			(47,100)			(47,100)
Common stock issued under employee stock plan, including tax benefits	$ 393	8,891			133	9,417
Common stock issued under employee stock plan, including tax benefits (in shares)	399,013
Stock-based compensation		3,993				3,993
Treasury stock purchased					(4,368)	$ (4,368)
Treasury stock purchased (in shares)	(123,000)					(100,000)
Treasury stock issued to benefit plan		572			959	$ 1,531
Treasury stock issued to benefit plan (in shares)	40,390
Balance at Dec. 31, 2014	$ 41,606	409,984	525,985	30,720	(20,391)	987,904
Balance (in shares) at Dec. 31, 2014	40,747,721
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			91,230			91,230
Other comprehensive income (loss), net of tax				(11,485)		(11,485)
Dividends declared:
Common			(50,624)			(50,624)
Common stock issued under employee stock plan, including tax benefits	$ 459	9,315				9,774
Common stock issued under employee stock plan, including tax benefits (in shares)	458,817
Stock-based compensation		4,201				4,201
Stock issued for acquisition	$ 2,378	99,824				102,202
Stock issued for acquisition (in shares)	2,377,329
Treasury stock purchased					(9,126)	$ (9,126)
Treasury stock purchased (in shares)	(265,230)					(300,000)
Treasury stock issued to benefit plan		4,691			11,880	$ 16,571
Treasury stock issued to benefit plan (in shares)	456,223
Balance at Dec. 31, 2015	$ 44,443	528,015	566,591	19,235	(17,637)	1,140,647
Balance (in shares) at Dec. 31, 2015	43,774,860
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			103,812			103,812
Other comprehensive income (loss), net of tax				(11,392)		(11,392)
Dividends declared:
Common			(55,711)			(55,711)
Common stock issued under employee stock plan, including tax benefits	$ 507	10,036				10,543
Common stock issued under employee stock plan, including tax benefits (in shares)	507,784
Stock-based compensation		4,783				4,783
Treasury stock purchased					(3,470)	$ (3,470)
Treasury stock purchased (in shares)	(67,826)					0
Treasury stock issued to benefit plan		2,941			5,947	$ 8,888
Treasury stock issued to benefit plan (in shares)	222,597
Balance at Dec. 31, 2016	$ 44,950	$ 545,775	$ 614,692	$ 7,843	$ (15,160)	$ 1,198,100
Balance (in shares) at Dec. 31, 2016	44,437,415