Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			$ 178,721	$ 77,259
Net income	$ 53,590	$ 151,770	111,636	232,382
Other comprehensive income	(581)	16,141	48,208	(46,350)
Common			(70,979)	(75,648)
Issuance of common stock			(3,783)	(7,346)
Conversion to common stock				0
Stock-based compensation expense			9,786	11,067
Repurchase of common stock			(194,776)
Balances, end of period	78,813	191,364	78,813	191,364
Preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			1	1
Conversion to common stock				0
Balances, end of period	1	1	1	1
Preference stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			505	548
Conversion to common stock			(16)	(26)
Balances, end of period	489	522	489	522
Common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			323,338	323,338
Balances, end of period	323,338	323,338	323,338	323,338
Additional paid-in capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			161,280	178,852
Issuance of common stock			(22,592)	(34,005)
Conversion to common stock			(320)	(543)
Stock-based compensation expense			9,786	11,067
Balances, end of period	148,154	155,371	148,154	155,371
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			5,155,537	4,897,708
Net income			111,636	232,382
Common			(70,979)	(75,648)
Balances, end of period	5,196,194	5,054,442	5,196,194	5,054,442
Accumulated other comprehensive loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			(888,635)	(846,156)
Other comprehensive income			48,208	(46,350)
Balances, end of period	(840,427)	(892,506)	(840,427)	(892,506)
Treasury stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balances, beginning of period			(4,573,305)	(4,477,032)
Issuance of common stock			18,809	26,659
Conversion to common stock			336	569
Repurchase of common stock			(194,776)
Balances, end of period	$ (4,748,936)	$ (4,449,804)	$ (4,748,936)	$ (4,449,804)