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Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Thousands | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 6,345,290 | $ 5,791,676
Equity securities at fair value | 121,653 | 101,864
Fair Value, Inputs, Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Equity securities at fair value | 303 |
Derivatives embedded in fixed indexed annuity products | $ 104,488 |
Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | lapse rate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0530 |
Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | Mortality multiplier |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.6300 |
Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | non-performance adjustment |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0500 |
Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | option budget |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0090 |
Minimum | Black-Scholes | Fair Value, Inputs, Level 3 | equity value |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Equity securities, measurement input | 0.3100 |
Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | option budget |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0250 |
Maximum | Black-Scholes | Fair Value, Inputs, Level 3 | equity value |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Equity securities, measurement input | 0.4100 |
Municipal bonds |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 1,827,497 | 1,686,203
Municipal bonds | Fair Value, Inputs, Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 59,590 |
Municipal bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | I spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0307 |
Municipal bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | I spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0391 |
Corporate bonds |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 2,122,899 | 1,581,424
Corporate bonds | Fair Value, Inputs, Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 155,816 |
Corporate bonds | Weighted Average | market comparable | Fair Value, Inputs, Level 3 | option adjusted spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.1254 |
Corporate bonds | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | N spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0272 |
Corporate bonds | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | N spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0553 |
Other asset-backed securities |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 1,231,850 | 1,295,492
Other asset-backed securities | Fair Value, Inputs, Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 128,283 |
Other asset-backed securities | Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | constant prepayment rate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.2000 |
Other asset-backed securities | Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | P D I interest margin |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0713 |
Other asset-backed securities | Weighted Average | discounted cash flow | Fair Value, Inputs, Level 3 | S B L interest margin |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0450 |
Other asset-backed securities | Minimum | discounted cash flow | Fair Value, Inputs, Level 3 | T spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0235 |
Other asset-backed securities | Minimum | vendor price | Fair Value, Inputs, Level 3 | haircut |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0300 |
Other asset-backed securities | Maximum | discounted cash flow | Fair Value, Inputs, Level 3 | T spread |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0800 |
Other asset-backed securities | Maximum | vendor price | Fair Value, Inputs, Level 3 | haircut |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0500 |
Government mortgage-backed securities |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 684,838 | $ 724,319
Government mortgage-backed securities | Fair Value, Inputs, Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair Value | $ 11,074 |
Government mortgage-backed securities | Minimum | vendor price | Fair Value, Inputs, Level 3 | haircut |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0300 |
Government mortgage-backed securities | Maximum | vendor price | Fair Value, Inputs, Level 3 | haircut |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0500 |

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