Fair Value of Financial Instruments - Level 3 Assets and Liabilities Measured at Fair Value with Corresponding Pricing Sources (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 5,780.1	$ 5,714.6
Equity securities	40.6	41.9
Fair Value
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	5,780.1	5,714.6
Equity securities	40.6	41.9
Financial Liabilities	77.1	78.9
Fair Value | Mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	669.1	689.0
Fair Value | Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,171.2	1,177.1
Fair Value | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,953.5	1,910.3
Fair Value | Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,665.5	1,602.4
Fair Value | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	463.5	465.3
Equity securities	3.6	3.8
Totals	467.1	469.1
Financial Liabilities	72.0	73.1
Fair Value | Level 3 | Mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0.0	0.0
Fair Value | Level 3 | Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	85.2	78.4
Fair Value | Level 3 | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	336.6	342.6
Fair Value | Level 3 | Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	41.7	44.3
Fair Value | Level 3 | Internal
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	6.9	6.7
Equity securities	2.6	2.6
Totals	9.5	9.3
Financial Liabilities	72.0	73.1
Fair Value | Level 3 | Internal | Mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0.0	0.0
Fair Value | Level 3 | Internal | Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0.0	0.0
Fair Value | Level 3 | Internal | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	6.9	6.7
Fair Value | Level 3 | Internal | Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0.0	0.0
Fair Value | Level 3 | External
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	456.6	458.6
Equity securities	1.0	1.2
Totals	457.6	459.8
Financial Liabilities	0.0	0.0
Fair Value | Level 3 | External | Mortgage-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	0.0	0.0
Fair Value | Level 3 | External | Municipal bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	85.2	78.4
Fair Value | Level 3 | External | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	329.7	335.9
Fair Value | Level 3 | External | Other asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 41.7	$ 44.3