Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Thousands	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale	$ 6,168,558		$ 5,791,676
Equity securities at fair value	102,303		101,864
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale	1,816,501		1,686,203
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale	1,983,483		1,581,424
Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale	1,236,788		1,295,492
Government mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale	$ 702,862		$ 724,319
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities at fair value		$ 88
Derivatives embedded in fixed indexed annuity products		$ 98,070
Level 3 | discounted cash flow | lapse rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input		0.0525
Level 3 | discounted cash flow | mortality multiplier | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input		0.6100
Level 3 | discounted cash flow | option budget | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.0100
Level 3 | discounted cash flow | option budget | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input	0.0250
Level 3 | discounted cash flow | Non-Performance Adjustment | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivatives embedded in fixed indexed annuity products, measurement input		0.0500
Level 3 | Black Scholes | equity value | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities, measurement input	43.27
Level 3 | Black Scholes | equity value | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities, measurement input	44.53
Level 3 | Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale		$ 65,671
Level 3 | Municipal bonds | discounted cash flow | I Spread | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input	0.0578
Level 3 | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale		121,185
Level 3 | Corporate bonds | discounted cash flow | N Spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input	0.0320
Level 3 | Corporate bonds | discounted cash flow | N Spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input	0.0818
Level 3 | Corporate bonds | market comparable | option adjusted spread | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input	0.1254
Level 3 | Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale		$ 128,599
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input	0.0235
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input	0.0800
Level 3 | Other asset-backed securities | discounted cash flow | constant prepayment rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.2000
Level 3 | Other asset-backed securities | discounted cash flow | PDI Interest Margin | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.0713
Level 3 | Other asset-backed securities | discounted cash flow | SBL Interest Margin | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.0450
Level 3 | Other asset-backed securities | vendor price | haircut | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.0300
Level 3 | Other asset-backed securities | vendor price | haircut | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.0500
Level 3 | Government mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, available-for-sale		$ 12,199
Level 3 | Government mortgage-backed securities | vendor price | haircut | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.0300
Level 3 | Government mortgage-backed securities | vendor price | haircut | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt securities, measurement input		0.0500