Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Basic earnings per share [Abstract]
Net income	$ 109,221	$ 113,654	$ 178,412	$ 246,730
Weighted average common shares outstanding (in shares)	340,678	339,705	340,411	339,406
Basic income per share (in dollars per share)	$ 0.32	$ 0.33	$ 0.52	$ 0.73
Diluted earnings per share [Abstract]
Net income	$ 109,221	$ 113,654	$ 178,412	$ 246,730
Effect of dilutive securities [Abstract]
Diluted income available to common shareholders	$ 116,688	$ 121,395	$ 195,161	$ 262,212
Weighted-average shares - Basic (in shares)	340,678	339,705	340,411	339,406
Weighted-average shares - Diluted (in shares)	446,139	439,127	450,354	439,200
Diluted income per share (in dollars per share)	$ 0.26	$ 0.28	$ 0.43	$ 0.60
Antidilutive securities (in shares)	0	28,900	0	28,900
Effective income tax rate (in hundredths)	35.00%		35.00%
Unvested Restricted Stock Units
Effect of dilutive securities [Abstract]
Dilutive securities - Share based compensation (in shares)	1,209	1,831	1,444	2,203
2% Convertible Senior Notes due 2020
Effect of dilutive securities [Abstract]
Dilutive securities	$ 1,982	$ 3,049	$ 3,964	$ 6,098
Dilutive securities (in shares)	71,917	71,917	71,917	71,917
Stated interest rate (in hundredths)	2.00%	2.00%	2.00%	2.00%	2.00%
5% Convertible Senior Notes due 2017
Effect of dilutive securities [Abstract]
Dilutive securities	$ 1,728	$ 4,692	$ 4,406	$ 9,384
Dilutive securities (in shares)	13,307	25,674	15,449	25,674
Antidilutive securities (in shares)			14,000
Stated interest rate (in hundredths)	5.00%	5.00%	5.00%	5.00%	5.00%
9% Convertible Junior Subordinated Debentures due 2063
Effect of dilutive securities [Abstract]
Dilutive securities	$ 3,757	$ 0	$ 8,379	$ 0
Dilutive securities (in shares)	19,028	0	21,133	0
Antidilutive securities (in shares)			9,800
Stated interest rate (in hundredths)	9.00%	9.00%	9.00%	9.00%