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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

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