CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Series A Preferred Stock	Series B Preferred Stock	Preferred Stock	Preferred Stock Series A Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Retained Earnings Series A Preferred Stock	Retained Earnings Series B Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of accounting principle	$ 165							$ 429		$ (264)
Beginning Balance (in shares) at Dec. 31, 2016				7,300,000		428,903,559	7,728,920
Beginning Balance, shares issued (in shares) at Dec. 31, 2016						436,632,479
Beginning Balance at Dec. 31, 2016	2,347,058			$ 565,000		$ 87,327	$ (67,484)	1,175,564	$ (8,671)	595,322
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	241,931									241,931
Other comprehensive income, net of tax	4,254								4,254
Total comprehensive income	$ 246,185
Cash dividends:
Cash dividends		$ (3,961)	$ (8,616)								$ (3,961)	$ (8,616)
Redemption of Series A Preferred Stock (in shares)					(3,300,000)
Redemption of Series A Preferred Stock		$ (165,000)			$ (165,000)
Dividend equivalent units related to employee stock-based compensation plans								96		(96)
Issuance of common shares (in shares)	5,652,886					5,652,886
Issuance of common shares	$ 16,467					$ 1,131		15,336
Stock-based compensation expense	$ 21,773							21,773
Shares repurchased related to employee stock-based compensation plans (in shares)	(2,666,781)					(2,666,781)	(2,666,781)
Shares repurchased related to employee stock-based compensation plans	$ (32,231)						$ (32,231)
Ending Balance (in shares) at Sep. 30, 2017				4,000,000		431,889,664	10,395,701
Ending Balance, shares issued (in shares) at Sep. 30, 2017						442,285,365
Ending Balance at Sep. 30, 2017	2,421,840			$ 400,000		$ 88,458	$ (99,715)	1,213,198	(4,417)	824,316
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of accounting principle | Accounting Standards Update 2017-12	1,052								270	782
Beginning Balance (in shares) at Dec. 31, 2017				4,000,000		432,376,250	11,087,337
Beginning Balance, shares issued (in shares) at Dec. 31, 2017						443,463,587
Beginning Balance at Dec. 31, 2017	2,474,256			$ 400,000		$ 88,693	$ (107,644)	1,222,277	2,748	868,182
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	339,964									339,964
Other comprehensive income, net of tax	23,402								23,402
Total comprehensive income	$ 363,366
Reclassification resulting from the adoption of ASU No. 2018-02									592	(592)
Cash dividends:
Cash dividends			$ (11,441)									$ (11,441)
Issuance of common shares (in shares)	6,341,950					6,341,950
Issuance of common shares	$ 21,927					$ 1,269		20,658
Stock-based compensation expense	$ 25,828							25,828
Shares repurchased related to employee stock-based compensation plans (in shares)	(3,056,746)					(3,056,746)	(3,056,746)
Shares repurchased related to employee stock-based compensation plans	$ (34,653)						$ (34,653)
Ending Balance (in shares) at Sep. 30, 2018				4,000,000		435,661,454	14,144,083
Ending Balance, shares issued (in shares) at Sep. 30, 2018						449,805,537
Ending Balance at Sep. 30, 2018	$ 2,840,335			$ 400,000		$ 89,962	$ (142,297)	$ 1,268,763	$ 27,012	$ 1,196,895