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, 2012	$ 40,421	$ 378,413	$ 447,018	$ 54,334	$ (17,408)	$ 902,778
Balance (in shares) at Dec. 31, 2012	39,625,933
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			78,829			78,829
Other comprehensive income (loss), net of tax				(80,880)		(80,880)
Dividends declared:
Common			(44,115)			(44,115)
Common stock issued under employee stock plan, including tax benefits	$ 792	14,154			293	15,239
Common stock issued under employee stock plan, including tax benefits (in shares)	805,385
Stock-based compensation		3,961				$ 3,961
Treasury stock purchased (in shares)						0
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