CONSOLIDATED STATEMENT OF CHANGES IN SHAREHOLDERS' EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	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, 2011	$ 37,795	$ 313,501	$ 411,805	$ 29,165	$ (17,683)	$ 774,583
Balance (in shares) at Dec. 31, 2011	36,986,409
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income			77,068			77,068
Other comprehensive income (loss), net of tax				25,169		25,169
Dividends declared:
Common			(41,855)			(41,855)
Common stock issued under employee stock plan, including tax benefits	496	8,457			275	9,228
Common stock issued under Employee stock plan, including tax benefits (in shares)	509,724
Stock-based compensation		3,668				3,668
Common stock issuance	2,130	52,787				54,917
Common stock issuance (in shares)	2,129,800
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
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	9,463			133	9,989
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)
Treasury stock sold					959	959
Treasury stock sold (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