Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic earnings per share:
Net income	$ 118,622	$ 109,221	$ 208,420	$ 178,412
Weighted average common shares outstanding - basic (in shares)	366,918	340,678	354,035	340,411
Basic earnings per share (in dollars per share)	$ 0.32	$ 0.32	$ 0.59	$ 0.52
Diluted earnings per share:
Net income	$ 118,622	$ 109,221	$ 208,420	$ 178,412
Diluted income available to common shareholders	$ 122,890	$ 116,688	$ 218,550	$ 195,161
Weighted average common shares outstanding - basic (in shares)	366,918	340,678	354,035	340,411
Effect of dilutive securities:
Weighted average common shares outstanding - diluted (in shares)	394,470	446,139	398,302	450,354
Diluted earnings per share (in dollars per share)	$ 0.31	$ 0.26	$ 0.55	$ 0.43
Effective income tax rate (in hundredths)	35.00%	35.00%	35.00%	35.00%
Unvested RSUs
Effect of dilutive securities:
Unvested RSUs (in shares)	1,140	1,209	1,314	1,444
2% Notes
Diluted earnings per share:
Dilutive securities	$ 84	$ 1,982	$ 907	$ 3,964
Effect of dilutive securities:
Dilutive securities (in shares)	3,827	71,917	16,771	71,917
Stated interest rate (in hundredths)	2.00%		2.00%
5% Notes
Diluted earnings per share:
Dilutive securities	$ 427	$ 1,728	$ 1,709	$ 4,406
Effect of dilutive securities:
Dilutive securities (in shares)	3,557	13,307	7,154	15,449
Stated interest rate (in hundredths)	5.00%		5.00%
9% Debentures
Diluted earnings per share:
Dilutive securities	$ 3,757	$ 3,757	$ 7,514	$ 8,379
Effect of dilutive securities:
Dilutive securities (in shares)	19,028	19,028	19,028	21,133
Stated interest rate (in hundredths)	9.00%		9.00%

Contractual interest rate for funds borrowed, under the debt agreement.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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