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Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Aug. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | --- | ---
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% |  |  |
Senior Notes | 5.25% Senior Notes due 2028 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Stated interest rate (in hundredths) | 5.25% | 5.25% |  |
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% |  | 9.00% | 9.00%
Carrying Value |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Other invested assets | $ 3,100 |  | $ 3,100 |
FHLB Advance | 155,000 |  | 155,000 |
9% Debentures | 208,814 |  | 256,872 |
Total financial liabilities | 1,243,193 |  | 832,739 |
Carrying Value | Senior Notes | 5.75% Senior Notes due 2023 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Senior Notes | 240,597 |  | 420,867 |
Carrying Value | Senior Notes | 5.25% Senior Notes due 2028 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Senior Notes | 638,782 |  | 0 |
Fair Value | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Other invested assets | 3,100 |  | 3,100 |
FHLB Advance | 160,865 |  | 156,422 |
9% Debentures | 273,569 |  | 346,289 |
Total financial liabilities | 1,392,635 |  | 974,538 |
Fair Value | Significant Other Observable Inputs (Level 2) | Senior Notes | 5.75% Senior Notes due 2023 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Senior Notes | 261,752 |  | 471,827 |
Fair Value | Significant Other Observable Inputs (Level 2) | Senior Notes | 5.25% Senior Notes due 2028 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Senior Notes | $ 696,449 |  | $ 0 |

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