Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Preferred Equity	Common Stock	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2014	$ 2,800,251	$ 59,727	$ 1,665	$ 1,484,933	$ 1,497,818	$ (4,850)	$ (239,042)
Preferred stock shares outstanding, beginning balance (in shares) at Dec. 31, 2014		62
Common stock shares outstanding, beginning balance (in shares) at Dec. 31, 2014			166,544
Comprehensive income
Net income	188,301	$ 0	$ 0	0	188,301	0	0
Other comprehensive income (loss)	(27,766)	0	0	0	0	(27,766)	0
Comprehensive income	160,535
Common stock issued
Stock-based compensation plans, net	20,054	$ 0	$ 0	3,316	(22,538)	0	39,276
Stock Issued During Period, Shares, Acquisitions		0	2,621
Stock Issued During Period, Value, Acquisitions	43,530	$ 0	$ 26	43,504	0	0	0
Purchase of common stock returned to authorized but unissued		0	(4,965)
Purchase of common stock returned to authorized but unissued	(93,000)	$ 0	$ (49)	(92,951)	0	0	0
Purchase of treasury stock	(5,154)	0	0	0	0	0	(5,154)
Cash dividends
Common stock dividends	(62,400)	0	0	0	(62,400)	0	0
Preferred stock dividends	(7,155)	$ 0	$ 0	0	(7,155)	0	0
Issuance of preferred stock		65	0
Issuance of preferred stock	62,966	$ 62,966	$ 0	0	0	0	0
Purchase of preferred stock		(2)	0
Purchase of preferred stock	(1,335)	$ (1,209)	$ 0	0	(126)	0	0
Other	(766)	$ (105)	$ 0	0	(661)	0	0
Stockholders' Equity, Other Shares		0	0
Stock-based compensation expense, net	18,202	$ 0	$ 0	18,202	0	0	0
Tax impact of stock-based compensation	1,518	0	0	1,518	0	0	0
Ending balance at Dec. 31, 2015	2,937,246	$ 121,379	$ 1,642	1,458,522	1,593,239	(32,616)	(204,920)
Common stock shares outstanding, ending balance (in shares) at Dec. 31, 2015			164,200
Preferred stock shares outstanding, ending balance (in shares) at Dec. 31, 2015		125
Comprehensive income
Net income	200,274	$ 0	$ 0	0	200,274	0	0
Other comprehensive income (loss)	(22,063)	0	0	0	0	(22,063)	0
Comprehensive income	178,211
Common stock issued
Stock-based compensation plans, net	21,748	$ 0	$ 0	1,940	(19,356)	0	39,164
Purchase of common stock returned to authorized but unissued		0	(1,170)
Purchase of common stock returned to authorized but unissued	(20,007)	$ 0	$ (12)	(19,995)	0	0	0
Purchase of treasury stock	(5,074)	0	0	0	0	0	(5,074)
Cash dividends
Common stock dividends	(67,855)	0	0	0	(67,855)	0	0
Preferred stock dividends	(8,903)	$ 0	$ 0	0	(8,903)	0	0
Issuance of preferred stock		100	0
Issuance of preferred stock	97,066	$ 97,066	$ 0	0	0	0	0
Redemption of preferred stock	(58,903)	$ (57,338)	$ 0	0	(1,565)	0	0
Redemption of preferred stock		(60)	0
Purchase of preferred stock		0	0
Purchase of preferred stock	(1,248)	$ (1,178)	$ 0	0	(70)	0	0
Stock-based compensation expense, net	21,971	0	0	21,971	0	0	0
Tax impact of stock-based compensation	(2,940)	0	0	(2,940)	0	0	0
Ending balance at Dec. 31, 2016	3,091,312	$ 159,929	$ 1,630	1,459,498	1,695,764	(54,679)	(170,830)
Common stock shares outstanding, ending balance (in shares) at Dec. 31, 2016			163,030
Preferred stock shares outstanding, ending balance (in shares) at Dec. 31, 2016		165
Comprehensive income
Net income	229,264	$ 0	$ 0	0	229,264	0	0
Other comprehensive income (loss)	(8,079)	0	0	0	0	(8,079)	0
Comprehensive income	221,185
Common stock issued
Stock-based compensation plans, net	27,619	$ 0	$ 0	2,297	(20,034)	0	45,356
Stock Issued During Period, Shares, Acquisitions		0	291
Stock Issued During Period, Value, Acquisitions	7,151	$ 0	$ 3	7,148	0	0	0
Purchase of common stock returned to authorized but unissued		0	(1,569)
Purchase of common stock returned to authorized but unissued	(37,031)	$ 0	$ (15)	(37,016)	0	0	0
Purchase of treasury stock	(9,290)	0	0	0	0	0	(9,290)
Cash dividends
Common stock dividends	(76,417)	0	0	0	(76,417)	0	0
Preferred stock dividends	(9,347)	0	0	0	(9,347)	0	0
Stock-based compensation expense, net	21,227	0	0	21,227	0	0	0
Tax impact of stock-based compensation	1,034	0	0	1,034	0	0	0
Ending balance at Dec. 31, 2017	$ 3,237,443	$ 159,929	$ 1,618	$ 1,454,188	$ 1,819,230	$ (62,758)	$ (134,764)
Common stock shares outstanding, ending balance (in shares) at Dec. 31, 2017			161,752
Preferred stock shares outstanding, ending balance (in shares) at Dec. 31, 2017		165