Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2012	$ 913,822	$ 116	$ 542,018	$ 750,697	$ (3,346)	$ (375,663)
Net income	284,501			284,501
Other comprehensive loss, net of tax	(64,471)				(64,471)
Issuance of common stock	89,650	1	89,649
Ending Balance at Dec. 31, 2013	1,223,502	117	631,667	1,035,198	(67,817)	(375,663)
Net income	368,707			368,707
Other comprehensive loss, net of tax	(223,691)				(223,691)
Issuance of treasury stock	1,125,964		1,096,698			29,266
Issuance of common stock	124,080	3	124,077
Ending Balance at Dec. 31, 2014	2,618,562	120	1,852,442	1,403,905	(291,508)	(346,397)
Net income	362,431			362,431
Other comprehensive loss, net of tax	(279,303)				(279,303)
Issuance of treasury stock	(8,119)					(8,119)
Issuance of common stock	136,476	1	136,475
Ending Balance at Dec. 31, 2015	$ 2,830,047	$ 121	$ 1,988,917	$ 1,766,336	$ (570,811)	$ (354,516)