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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

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- Definition The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(19)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96947427&loc=d3e1252-109256 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(21)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(23)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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