Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	$ 6,512.0	$ 6,345.3
Equity securities at fair value	152.3	121.6
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	1,775.5	1,827.5
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	2,451.6	2,122.9
Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, available-for-sale	1,138.5	$ 1,231.8
Level 3
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products	$ 106.7
Level 3 | discounted cash flow | lapse rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.0530
Level 3 | discounted cash flow | mortality multiplier | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.6300
Level 3 | discounted cash flow | option budget | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.0090
Level 3 | discounted cash flow | option budget | Maximum
Fair Value Measurement Inputs and Valuation Techniques
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
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
Equity securities, measurement input	0.2800
Level 3 | Black Scholes | equity value | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Equity securities, measurement input	0.3500
Level 3 | Municipal bonds | discounted cash flow | I Spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0307
Level 3 | Municipal bonds | discounted cash flow | I Spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0391
Level 3 | Corporate bonds | discounted cash flow | N Spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0272
Level 3 | Corporate bonds | discounted cash flow | N Spread | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0553
Level 3 | Corporate bonds | market comparable | option adjusted spread | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.1254
Level 3 | Other asset-backed securities | discounted cash flow | constant prepayment rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.2000
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Minimum
Fair Value Measurement Inputs and Valuation Techniques
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
Debt securities, measurement input	0.0800
Level 3 | Other asset-backed securities | discounted cash flow | PDI interest margin | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques
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
Debt securities, measurement input	0.0450
Level 3 | Other asset-backed securities | vendor price | haircut | Minimum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0300
Level 3 | Other asset-backed securities | vendor price | haircut | Maximum
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0500