Investments - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	$ 185.7	$ 200.5
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	$ 1,537.3
Debt Securities, Available-for-Sale, Unrealized Loss Amount, Average, Percentage	0.049
Financing Receivable, Accrued Interest, before Allowance for Credit Loss	$ 70.9	77.0
Equity Securities, FV-NI, Current	218.5	225.8
Equity securities, FV-NI, unrealized gain (loss)	(5.1)	3.0
Convertible Securities at Fair Value	280.7	281.2
Debt and equity securities, gain (loss), excluding other-than-temporary impairment loss	3.2	8.0
Outstanding commitments to fund equity method limited liability investments	$ 91.3
Equity method investments, reported without a reporting lag	0.031
Equity method investments, reported with a one-moth reporting lag	0.041
Equity Securities at Modified Cost	$ 539.2	513.5
Other Asset Impairment Charges	(5.8)	(1.1)	$ (25.8)
Other Investments and Securities, at Cost	22.5	32.6
Alternative Energy Partnership Investments
Debt and Equity Securities, FV-NI [Line Items]
Equity Method Investments	17.6	17.3
Equity Method Limited Liability Investments
Debt and Equity Securities, FV-NI [Line Items]
Equity Method Investments	186.3	221.7
Dividends on Equity Securities Excluding Alternative Investments
Debt and Equity Securities, FV-NI [Line Items]
Other Asset Impairment Charges	(0.4)	(0.5)	0.0
Other than Temporary Impairment Losses, Investments			$ 0.0
Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	1.8	0.9
Other liabilities	11.6	0.0
Debt securities, available-for-sale, unrealized loss position, accumulated loss	889.5	730.3
Government National Mortgage Association Certificates and Obligations (GNMA)
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	207.0
Federal National Mortgage Association Certificates and Obligations (FNMA)
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	99.5
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	76.3
Other Non-Governmental Issuers
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,154.5
Foreign Governments
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	0.2	0.6
Bonds and Notes
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	518.8	383.8
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	0.0	0.0
Other liabilities	0.0	0.0
Dividends on Equity Securities Excluding Alternative Investments
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	0.3	0.1
Other liabilities	0.0	0.0
Investment-grade | Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	875.3	704.8
Non-investment-grade | Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 14.2	$ 25.5
Percentage of unrealized loss position to amortized cost basis of available for sale security average		8.80%