Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Participating securities (as shares)									100	100	100
Basic earnings per share [Abstract]
Net income	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 102,418	$ 822,852	$ 113,654	$ 133,076	$ 342,517	$ 1,172,000	$ 251,949
Weighted average common shares outstanding - basic (in shares)									342,890	339,552	338,523
Basic income per share (in dollars per share)	$ 0.31	$ 0.16	$ 0.32	$ 0.20	$ 0.30	$ 2.42	$ 0.33	$ 0.39	$ 1.00	$ 3.45	$ 0.74
Diluted earnings per share [Abstract]
Net income	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 102,418	$ 822,852	$ 113,654	$ 133,076	$ 342,517	$ 1,172,000	$ 251,949
Diluted income available to common shareholders									$ 370,883	$ 1,214,942	$ 264,146
Weighted-average shares - Basic (in shares)									342,890	339,552	338,523
Effect of dilutive securities [Abstract]
Weighted-average shares - Diluted (in shares)									431,992	468,039	413,522
Diluted income per share (in dollars per share)	$ 0.28	$ 0.14	$ 0.26	$ 0.17	$ 0.24	$ 1.78	$ 0.28	$ 0.32	$ 0.86	$ 2.60	$ 0.64
Antidilutive securities (in shares)									0	0	54,500
Federal statutory income tax rate (in hundredths)									35.00%	35.00%	35.00%
2% Convertible Senior Notes due 2020
Diluted earnings per share [Abstract]
Dilutive securities									$ 6,111	$ 7,928	$ 12,197
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									54,450	71,917	71,917
Stated interest rate (in hundredths)	2.00%								2.00%
5% Convertible Senior Notes due 2017
Diluted earnings per share [Abstract]
Dilutive securities									$ 6,362	$ 12,228	$ 0
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									13,107	25,603	0
Stated interest rate (in hundredths)	5.00%								5.00%
9% Convertible Junior Subordinated Debentures due 2063
Diluted earnings per share [Abstract]
Dilutive securities									$ 15,893	$ 22,786	$ 0
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									20,075	28,854	0
Stated interest rate (in hundredths)	9.00%								9.00%
Unvested Restricted Stock Units
Effect of dilutive securities [Abstract]
Dilutive securities - Share based compensation (in shares)									1,470	2,113	3,082