Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 5,345.4		$ 5,235.3
Equity securities at fair value, (cost $101700000 and $86200000)	82.6		86.2
Net MRBs	(6.2)	$ (6.3)	(3.9)	$ (3.6)	$ 1.3	$ 0.2
Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	5,345.4		5,235.3
Equity securities at fair value, (cost $101700000 and $86200000)	82.6		86.2
Financial Liabilities	81.8		86.0
Mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	669.2		653.2
Mortgage-backed securities | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	669.2		653.2
Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,176.7		1,270.1
Municipal bonds | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,176.7		1,270.1
Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,846.2		1,772.7
Corporate bonds | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,846.2		1,772.7
Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,257.8		1,128.4
Other asset-backed securities | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	1,257.8		1,128.4
Level 3 | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	523.9		514.0
Equity securities at fair value, (cost $101700000 and $86200000)	2.7		4.5
Totals	526.6		518.5
Financial Liabilities	77.9		82.4
Derivatives embedded in fixed indexed annuity products	84.1
Net MRBs	(6.2)
Level 3 | Fair Value | Recurring | Internal
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	219.2		180.4
Equity securities at fair value, (cost $101700000 and $86200000)	0.0		0.0
Totals	219.2		180.4
Financial Liabilities	77.9		82.4
Level 3 | Fair Value | Recurring | External
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	304.7		333.6
Equity securities at fair value, (cost $101700000 and $86200000)	2.7		4.5
Totals	307.4		338.1
Financial Liabilities	0.0		0.0
Level 3 | Mortgage-backed securities | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	0.0		0.0
Level 3 | Mortgage-backed securities | Fair Value | Recurring | Internal
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	0.0		0.0
Level 3 | Mortgage-backed securities | Fair Value | Recurring | External
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	0.0		0.0
Level 3 | Municipal bonds | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	72.1		74.0
Level 3 | Municipal bonds | Fair Value | Recurring | Internal
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	0.0		0.0
Level 3 | Municipal bonds | Fair Value | Recurring | External
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	72.1		74.0
Level 3 | Corporate bonds | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	366.4		342.5
Level 3 | Corporate bonds | Fair Value | Recurring | Internal
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	219.2		180.4
Level 3 | Corporate bonds | Fair Value | Recurring | External
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	147.2		162.1
Level 3 | Other asset-backed securities | Fair Value | Recurring
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	85.4		97.5
Level 3 | Other asset-backed securities | Fair Value | Recurring | Internal
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	0.0		0.0
Level 3 | Other asset-backed securities | Fair Value | Recurring | External
Fair Value Measurement Inputs and Valuation Techniques
Fair Value	$ 85.4		$ 97.5
Minimum | Level 3 | discounted cash flow | option budget
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.009
Minimum | Level 3 | Corporate bonds | discounted cash flow | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.046
Minimum | Level 3 | Corporate bonds | discounted cash flow | option adjusted spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0313
Maximum | Level 3 | discounted cash flow | option budget
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.038
Maximum | Level 3 | Corporate bonds | discounted cash flow | yield
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.164
Maximum | Level 3 | Corporate bonds | discounted cash flow | option adjusted spread
Fair Value Measurement Inputs and Valuation Techniques
Debt securities, measurement input	0.0839
Weighted Average | Level 3 | discounted cash flow | lapse rate
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.059
Net MRBs, measurement input	0.059
Weighted Average | Level 3 | discounted cash flow | mortality multiplier
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.694
Net MRBs, measurement input	0.694
Weighted Average | Level 3 | discounted cash flow | non-performance adjustment
Fair Value Measurement Inputs and Valuation Techniques
Derivatives embedded in fixed indexed annuity products, measurement input	0.050