Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Basic earnings per share:
Net income	$ 89,798	$ 69,191
Weighted average common shares outstanding (in shares)	341,009	340,144
Basic income per share (in dollars per share)	$ 0.26	$ 0.20
Diluted earnings per share:
Net income	$ 89,798	$ 69,191
Diluted income available to common shareholders	$ 95,660	$ 73,851
Weighted-average shares - Basic (in shares)	341,009	340,144
Effect of dilutive securities:
Weighted-average shares - Diluted (in shares)	402,175	431,365
Diluted income per share (in dollars per share)	$ 0.24	$ 0.17
Antidilutive securities (in shares)	0	23,300
Effective income tax rate (in hundredths)	35.00%	35.00%
2% Notes
Diluted earnings per share:
Dilutive securities	$ 823	$ 1,982
Effect of dilutive securities:
Dilutive securities (in shares)	29,859	71,917
Stated interest rate (in hundredths)	2.00%
5% Convertible Senior Notes due 2017
Diluted earnings per share:
Dilutive securities	$ 1,282	$ 2,678
Effect of dilutive securities:
Dilutive securities (in shares)	10,791	17,625
Stated interest rate (in hundredths)	5.00%
9% Convertible Junior Subordinated Debentures due 2063
Diluted earnings per share:
Dilutive securities	$ 3,757	$ 0
Effect of dilutive securities:
Dilutive securities (in shares)	19,028	0
Stated interest rate (in hundredths)	9.00%
Unvested Restricted Stock Units
Effect of dilutive securities:
Unvested RSUs (in shares)	1,488	1,679