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Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per share [Abstract]
Net income	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 355,761	$ 342,517	$ 1,172,000
Weighted average common shares outstanding - basic (in shares)									362,380	342,890	339,552
Basic income per share (in dollars per share)	$ 0.07	$ 0.32	$ 0.32	$ 0.26	$ 0.31	$ 0.16	$ 0.32	$ 0.20	$ 0.98	$ 1.00	$ 3.45
Diluted earnings per share [Abstract]
Net income	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 355,761	$ 342,517	$ 1,172,000
Diluted income available to common shareholders									$ 373,404	$ 370,883	$ 1,214,942
Weighted-average shares - Basic (in shares)									362,380	342,890	339,552
Effect of dilutive securities [Abstract]
Weighted-average shares - Diluted (in shares)									394,766	431,992	468,039
Diluted income per share (in dollars per share)	$ 0.07	$ 0.32	$ 0.31	$ 0.24	$ 0.28	$ 0.14	$ 0.26	$ 0.17	$ 0.95	$ 0.86	$ 2.60
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									$ 907	$ 6,111	$ 7,928
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									8,317	54,450	71,917
Stated interest rate (in hundredths)	2.00%								2.00%
5% Convertible Senior Notes due 2017
Diluted earnings per share [Abstract]
Dilutive securities									$ 1,709	$ 6,362	$ 12,228
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									3,548	13,107	25,603
Stated interest rate (in hundredths)	5.00%								5.00%
9% Convertible Junior Subordinated Debentures due 2063
Diluted earnings per share [Abstract]
Dilutive securities									$ 15,027	$ 15,893	$ 22,786
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									19,028	20,075	28,854
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)									1,493	1,470	2,113