Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic earnings per share:
Net income	$ 120,027	$ 56,618	$ 328,447	$ 235,030
Weighted average common shares outstanding - basic (in shares)	370,586	349,376	359,613	343,403
Basic earnings per share (in dollars per share)	$ 0.32	$ 0.16	$ 0.91	$ 0.68
Diluted earnings per share:
Net income	$ 120,027	$ 56,618	$ 328,447	$ 235,030
Diluted income available to common shareholders	$ 123,784	$ 58,615	$ 342,333	$ 245,398
Weighted average common shares outstanding - basic (in shares)	370,586	349,376	359,613	343,403
Effect of dilutive securities:
Weighted average common shares outstanding - diluted (in shares)	391,087	406,050	395,870	421,423
Diluted earnings per share (in dollars per share)	$ 0.32	$ 0.14	$ 0.86	$ 0.58
Antidilutive securities (in shares)	0	19,000	0	20,400
Effective income tax rate (in hundredths)	35.00%	35.00%	35.00%	35.00%
Unvested RSUs
Effect of dilutive securities:
Unvested RSUs (in shares)	1,473	1,395	1,367	1,428
2% Notes
Diluted earnings per share:
Dilutive securities	$ 0	$ 1,324	$ 907	$ 5,288
Effect of dilutive securities:
Dilutive securities (in shares)	0	44,488	11,119	62,707
Stated interest rate (in hundredths)	2.00%		2.00%
5% Notes
Diluted earnings per share:
Dilutive securities	$ 0	$ 673	$ 1,709	$ 5,080
Effect of dilutive securities:
Dilutive securities (in shares)	0	10,791	4,743	13,885
Stated interest rate (in hundredths)	5.00%		5.00%
9% Debentures
Diluted earnings per share:
Dilutive securities	$ 3,757	$ 0	$ 11,270	$ 0
Effect of dilutive securities:
Dilutive securities (in shares)	19,028	0	19,028	0
Stated interest rate (in hundredths)	9.00%		9.00%