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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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic earnings per share [Abstract]
Net income	$ 157,746	$ 181,900	$ 186,814	$ 143,637	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 670,097	$ 355,761	$ 342,517
Weighted average common shares outstanding - basic (in shares)									365,406	362,380	342,890
Basic earnings per share (in dollars per share)	$ 0.44	$ 0.50	$ 0.51	$ 0.39	$ 0.07	$ 0.32	$ 0.32	$ 0.26	$ 1.83	$ 0.98	$ 1.00
Diluted earnings per share [Abstract]
Net income	$ 157,746	$ 181,900	$ 186,814	$ 143,637	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 670,097	$ 355,761	$ 342,517
Diluted income available to common shareholders									$ 688,361	$ 373,404	$ 370,883
Weighted-average shares - basic (in shares)									365,406	362,380	342,890
Effect of dilutive securities [Abstract]
Weighted-average shares - diluted (in shares)									386,078	394,766	431,992
Diluted income per share (in dollars per share)	$ 0.43	$ 0.49	$ 0.49	$ 0.38	$ 0.07	$ 0.32	$ 0.31	$ 0.24	$ 1.78	$ 0.95	$ 0.86
Federal statutory income tax rate (in hundredths)									21.00%	35.00%	35.00%
2% Convertible Senior Notes due 2020
Diluted earnings per share [Abstract]
Dilutive securities									$ 0	$ 907	$ 6,111
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									0	8,317	54,450
Stated interest rate (in hundredths)					2.00%					2.00%
5% Convertible Senior Notes due 2017
Diluted earnings per share [Abstract]
Dilutive securities									$ 0	$ 1,709	$ 6,362
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									0	3,548	13,107
Stated interest rate (in hundredths)					5.00%					5.00%
9% Convertible Junior Subordinated Debentures due 2063
Diluted earnings per share [Abstract]
Dilutive securities									$ 18,264	$ 15,027	$ 15,893
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									19,028	19,028	20,075
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)									1,644	1,493	1,470