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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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic earnings per share [Abstract]
Net income	$ 177,110	$ 176,934	$ 167,778	$ 151,941	$ 157,746	$ 181,900	$ 186,814	$ 143,637	$ 673,763	$ 670,097	$ 355,761
Weighted average common shares outstanding - basic (in shares)									352,827	365,406	362,380
Basic earnings per share (in dollars per share)	$ 0.51	$ 0.50	$ 0.47	$ 0.43	$ 0.44	$ 0.50	$ 0.51	$ 0.39	$ 1.91	$ 1.83	$ 0.98
Diluted earnings per share [Abstract]
Net income	$ 177,110	$ 176,934	$ 167,778	$ 151,941	$ 157,746	$ 181,900	$ 186,814	$ 143,637	$ 673,763	$ 670,097	$ 355,761
Diluted income available to common shareholders									$ 692,027	$ 688,361	$ 373,404
Weighted-average shares - basic (in shares)									352,827	365,406	362,380
Effect of dilutive securities [Abstract]
Weighted-average shares - diluted (in shares)									373,924	386,078	394,766
Diluted income per share (in dollars per share)	$ 0.49	$ 0.49	$ 0.46	$ 0.42	$ 0.43	$ 0.49	$ 0.49	$ 0.38	$ 1.85	$ 1.78	$ 0.95
Federal statutory income tax rate (in hundredths)									21.00%	21.00%	35.00%
2% Convertible Senior Notes due 2020
Diluted earnings per share [Abstract]
Dilutive securities									$ 0	$ 0	$ 907
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									0	0	8,317
Stated interest rate (in hundredths)											2.00%
5% Convertible Senior Notes due 2017
Diluted earnings per share [Abstract]
Dilutive securities									$ 0	$ 0	$ 1,709
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									0	0	3,548
Stated interest rate (in hundredths)											5.00%
9% Convertible Junior Subordinated Debentures due 2063
Diluted earnings per share [Abstract]
Dilutive securities									$ 18,264	$ 18,264	$ 15,027
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)									19,028	19,028	19,028
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)									2,069	1,644	1,493