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Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic earnings per share:
Net income	$ 181,900	$ 120,027	$ 512,351	$ 328,447
Weighted average common shares outstanding - basic (in shares)	362,180	370,586	367,190	359,613
Basic earnings per share (in dollars per share)	$ 0.50	$ 0.32	$ 1.40	$ 0.91
Diluted earnings per share:
Net income	$ 181,900	$ 120,027	$ 512,351	$ 328,447
Diluted income available to common shareholders	$ 186,466	$ 123,784	$ 526,049	$ 342,333
Weighted average common shares outstanding - basic (in shares)	362,180	370,586	367,190	359,613
Effect of dilutive securities:
Weighted average common shares outstanding - diluted (in shares)	382,905	391,087	387,765	395,870
Diluted earnings per share (in dollars per share)	$ 0.49	$ 0.32	$ 1.36	$ 0.86
Effective income tax rate			21.00%	35.00%
2% Convertible Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stated interest rate		2.00%		2.00%
Diluted earnings per share:
Dilutive securities	$ 0	$ 0	$ 0	$ 907
Effect of dilutive securities:
Dilutive securities (in shares)	0	0	0	11,119
5% Convertible Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stated interest rate		5.00%		5.00%
Diluted earnings per share:
Dilutive securities	$ 0	$ 0	$ 0	$ 1,709
Effect of dilutive securities:
Dilutive securities (in shares)	0	0	0	4,743
9% Convertible Junior Subordinated Debentures
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stated interest rate	9.00%		9.00%
Diluted earnings per share:
Dilutive securities	$ 4,566	$ 3,757	$ 13,698	$ 11,270
Effect of dilutive securities:
Dilutive securities (in shares)	19,028	19,028	19,028	19,028
Unvested RSUs
Effect of dilutive securities:
Unvested RSUs (in shares)	1,697	1,473	1,547	1,367

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