CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Series A Preferred Stock	Series B Preferred Stock	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
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	94,943								94,943
Other comprehensive income (loss), net of tax	1,980							1,980
Total comprehensive income	96,923
Cash dividends:
Cash dividends		$ (2,875)	$ (2,700)							$ (2,875)	$ (2,700)
Dividend equivalent units related to employee stock-based compensation plans	$ 0						96		(96)
Issuance of common shares (in shares)	3,738,717				3,738,717
Issuance of common shares	$ 6,535				$ 748		5,787
Tax benefit related to employee stock-based compensation	0						0
Stock-based compensation expense	$ 9,425						9,425
Shares repurchased related to employee stock-based compensation plans (in shares)	(1,603,487)				(1,603,487)	(1,603,487)
Shares repurchased related to employee stock-based compensation plans	$ (19,175)					$ (19,175)
Ending Balance (in shares) at Mar. 31, 2017				7,300,000	431,038,789	(9,332,407)
Ending Balance, shares issued (in shares) at Mar. 31, 2017					440,371,196
Ending Balance at Mar. 31, 2017	2,435,356			$ 565,000	$ 88,075	$ (86,659)	1,191,466	(6,691)	684,165
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of the adoption of the new stock compensation standard amendment | Accounting Standards Update 2016-09, New Stock Compensation Standard Amendment	165						594		(429)
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	126,254								126,254
Other comprehensive income (loss), net of tax	12,261							12,261
Total comprehensive income	138,515
Reclassification resulting from the adoption of ASU No. 2018-02 | Accounting Standards Update 2018-02	$ 0							592	(592)
Cash dividends:
Cash dividends			$ (3,397)								$ (3,397)
Issuance of common shares (in shares)	5,559,991				5,559,991
Issuance of common shares	$ 16,699				$ 1,112		15,587
Stock-based compensation expense	$ 14,745						14,745
Shares repurchased related to employee stock-based compensation plans (in shares)	(2,740,018)				(2,740,018)	(2,740,018)
Shares repurchased related to employee stock-based compensation plans	$ (30,985)					$ (30,985)
Ending Balance (in shares) at Mar. 31, 2018				4,000,000	435,196,223	(13,827,355)
Ending Balance, shares issued (in shares) at Mar. 31, 2018					449,023,578
Ending Balance at Mar. 31, 2018	$ 2,609,833			$ 400,000	$ 89,805	$ (138,629)	$ 1,252,609	$ 15,601	$ 990,447