Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Series A Preferred Stock [Member]	Series E Preferred Stock [Member]	Series F Preferred Stock [Member]	Preferred Stock [Member]	Preferred Stock [Member] Series E Preferred Stock [Member]	Preferred Stock [Member] Series F Preferred Stock [Member]	Common Stock [Member]	Paid-In Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Series A Preferred Stock [Member]	Retained Earnings [Member] Series E Preferred Stock [Member]	Retained Earnings [Member] Series F Preferred Stock [Member]	Treasury Stock, at cost [Member]	AOCI Attributable to Parent [Member]
Beginning Balance at Dec. 31, 2014	$ 2,322,815				$ 151,649			$ 936	$ 1,127,534	$ 1,202,251				$ (103,294)	$ (56,261)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	204,729									204,729
Other comprehensive income (loss), net of tax	(21,845)														(21,845)
Dividends and dividend equivalents declared on common stock	(81,197)								119	(81,316)
Dividends on preferred stock		$ (615)	$ (8,096)								$ (615)	$ (8,096)
Common stock issued	0							1	(1)	0				0
Preferred stock conversion					(28,939)				(3,429)					32,368
Stock-based compensation, net of tax impact	12,947								2,906	(1,005)				11,046
Exercise of stock options	3,060								(2,781)					5,841
Common shares acquired related to stock compensation plan activity	(5,251)													(5,251)
Common stock repurchase program	(12,564)													(12,564)
Common stock warrants repurchased	(23)								(23)
Ending Balance at Dec. 31, 2015	2,413,960				122,710			937	1,124,325	1,315,948				(71,854)	(78,106)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	207,127									207,127
Other comprehensive income (loss), net of tax	1,113														1,113
Dividends and dividend equivalents declared on common stock	(89,913)								149	(90,062)
Dividends on preferred stock			(8,096)									(8,096)
Stock-based compensation, net of tax impact	14,092								2,976	403				10,713
Exercise of stock options	11,762								(1,350)					13,112
Common shares acquired related to stock compensation plan activity	(11,664)													(11,664)
Common stock repurchase program	(11,206)													(11,206)
Common stock warrants repurchased	(163)								(163)
Ending Balance at Dec. 31, 2016	2,527,012				122,710			937	1,125,937	1,425,320				(70,899)	(76,993)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	255,439									255,439
Other comprehensive income (loss), net of tax	1,110														1,110
Dividends and dividend equivalents declared on common stock	(94,929)								168	(95,097)
Dividends on preferred stock			(8,096)	$ (88)								$ (8,096)	$ (88)
Common stock issued				$ 145,056			$ 145,056
Stock-based compensation, net of tax impact	14,184								0	2,636				11,548
Exercise of stock options	8,259								(3,941)					12,200
Common shares acquired related to stock compensation plan activity	(11,694)													(11,694)
Common stock repurchase program	(11,585)													(11,585)
Stock Redeemed or Called During Period, Value			$ (122,710)			$ (122,710)
Ending Balance at Dec. 31, 2017	$ 2,701,958				$ 145,056			$ 937	$ 1,122,164	1,595,762				$ (70,430)	(91,531)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption										$ 15,648					$ (15,648)