Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Series C Preferred Stock	Preferred Stock Series C Preferred Stock	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Retained Earnings Series C Preferred Stock
Balance at Dec. 31, 2014	$ 3,041,270		$ 125,980	$ 139,950	$ 2,960,825	$ (187,774)	$ (12,605)	$ 14,894
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	226,082							226,082
Net current period other comprehensive income (loss)	(17,214)						(17,214)
Cash dividends declared on common stock	(55,354)							(55,354)
Cash dividends paid on Series C Preferred Stock		$ (10,238)							$ (10,238)
Repurchases of common stock	(199,221)				14,516	(213,737)
Restricted share unit activity	(4,984)			304	(4,877)			(411)
Stock options exercised	5,643			338	5,305
Share-based compensation tax benefit (deficiency)	1,656				1,656
Share-based compensation expense	12,556				12,556
Balance at Dec. 31, 2015	3,000,196		125,980	140,592	2,989,981	(401,511)	(29,819)	174,973
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	246,784							246,784
Net current period other comprehensive income (loss)	(25,840)						(25,840)
Cash dividends declared on common stock	(59,425)							(59,425)
Cash dividends paid on Series C Preferred Stock		(10,238)							(10,238)
Repurchases of common stock	(263,084)					(263,084)
Issuance of common stock	26,592			821	25,771
Restricted share unit activity	(5,041)			316	(5,030)			(327)
Stock options exercised	5,155			297	4,858
Share-based compensation tax benefit (deficiency)	(790)				(790)
Share-based compensation expense	13,615				13,615
Balance at Dec. 31, 2016	2,927,924		125,980	142,026	3,028,405	(664,595)	(55,659)	351,767
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	275,474							275,474
Net current period other comprehensive income (loss)	905						905
Cash dividends declared on common stock	(72,506)							(72,506)
Cash dividends paid on Series C Preferred Stock		$ (10,238)							$ (10,238)
Repurchases of common stock	(175,079)					(175,079)
Issuance of common stock	5,460			118	5,342
Restricted share unit activity	(7,993)			336	(8,039)			(290)
Stock options exercised	3,440			198	3,242
Share-based compensation tax benefit (deficiency)	5,000
Share-based compensation expense	14,179				14,179
Balance at Dec. 31, 2017	$ 2,961,566		$ 125,980	$ 142,678	$ 3,043,129	$ (839,674)	$ (54,754)	$ 544,207