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	Retained Earnings	Retained Earnings Series A Preferred Stock	Retained Earnings Series B Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of the adoption of the stock compensation standard amendment	$ 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	165,560									165,560
Other comprehensive income, net of tax	819								819
Total comprehensive income	166,379
Cash dividends:
Cash dividends		$ (3,961)	$ (5,588)								$ (3,961)	$ (5,588)
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	$ 0							96		(96)
Issuance of common shares (in shares)	5,229,774					5,229,774
Issuance of common shares	$ 14,494					$ 1,046		13,448
Stock-based compensation expense	$ 15,500							15,500
Shares repurchased related to employee stock-based compensation plans (in shares)	(2,584,964)					(2,584,964)	(2,584,964)
Shares repurchased related to employee stock-based compensation plans	$ (31,337)						$ (31,337)
Ending Balance (in shares) at Jun. 30, 2017				4,000,000		431,548,369	10,313,884
Ending Balance, shares issued (in shares) at Jun. 30, 2017						441,862,253
Ending Balance at Jun. 30, 2017	2,337,710			$ 400,000		$ 88,373	$ (98,821)	1,205,037	(7,852)	750,973
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	236,086									236,086
Other comprehensive income, net of tax	19,876								19,876
Total comprehensive income	$ 255,962
Reclassification resulting from the adoption of ASU No. 2018-02 | Accounting Standards Update 2018-02									592	(592)
Cash dividends:
Cash dividends			$ (7,317)									$ (7,317)
Issuance of common shares (in shares)	5,944,946					5,944,946
Issuance of common shares	$ 18,852					$ 1,189		17,663
Stock-based compensation expense	$ 20,261							20,261
Shares repurchased related to employee stock-based compensation plans (in shares)	(2,940,595)					(2,940,595)	(2,940,595)
Shares repurchased related to employee stock-based compensation plans	$ (33,312)						$ (33,312)
Ending Balance (in shares) at Jun. 30, 2018				4,000,000		435,380,601	14,027,932
Ending Balance, shares issued (in shares) at Jun. 30, 2018						449,408,533
Ending Balance at Jun. 30, 2018	$ 2,728,702			$ 400,000		$ 89,882	$ (140,956)	$ 1,260,201	$ 23,216	$ 1,096,359