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Fair Value Measurements - Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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5% Convertible Senior Notes due 2017 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Stated interest rate (in hundredths) | 5.00% |
2% Convertible Senior Notes due 2020 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Stated interest rate (in hundredths) | 2.00% |
Senior Notes | 5.75% Senior Notes due 2023 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Stated interest rate (in hundredths) | 5.75% |
9% Convertible Junior Subordinated Debentures due 2063 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Stated interest rate (in hundredths) | 9.00% |
Carrying Value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
FHLB Advance | $ 155,000 | $ 155,000
5% Notes | 0 | 144,789
2% Notes | 0 | 204,672
5.75% Notes | 418,560 | 417,406
9% Debentures | 256,872 | 256,872
Total financial liabilities | 830,432 | 1,178,739
Fair Value | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
FHLB Advance | 152,124 | 151,905
5% Notes | 0 | 147,679
2% Notes | 0 | 308,605
5.75% Notes | 465,473 | 445,987
9% Debentures | 353,507 | 323,040
Total financial liabilities | $ 971,104 | $ 1,377,216

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