Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
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