Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock
Beginning Balance at Dec. 31, 2014	$ 2,618,562	$ 120	$ 1,852,442	$ 1,403,905	$ (291,508)	$ (346,397)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	362,431			362,431
Other comprehensive income (loss), net of tax	(279,303)				(279,303)
Acquisition of common stock	(8,119)		0			(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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	452,385			452,385
Other comprehensive income (loss), net of tax	(95,592)				(95,592)
Acquisition of common stock	(187,979)		0			(187,979)
Issuance of common stock	85,177	0	85,177
Ending Balance at Dec. 31, 2016	3,084,038	121	2,074,094	2,218,721	(666,403)	(542,495)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	740,200			740,200
Other comprehensive income (loss), net of tax	114,546				114,546
Acquisition of common stock	(402,393)					(402,393)
Issuance of common stock	140,131	1	140,130
Ending Balance at Dec. 31, 2017	$ 3,676,522	$ 122	$ 2,214,224	$ 2,958,921	$ (551,857)	$ (944,888)