Investments - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)		Dec. 31, 2021 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	$ 200.5	$ 262.6	[1]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,660.2
Unrealized loss position for 12 months or longer		0.0
Equity securities, FV-NI, unrealized gain (loss)	3.0
Debt and equity securities, gain (loss), excluding other-than-temporary impairment loss	8.0
Total Assets	12,742.7	13,313.6	[1]
Liabilities	10,237.7	10,643.0	[2]
Net Loss	(272.1)	(286.6)	[3]	$ (123.7)	[3]
Outstanding commitments to fund equity method limited liability investments	85.3
Allowance for credit losses	(8.2)	(9.6)		(7.5)
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2.9	7.0
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.9)	5.8
Write-offs Charged Against Allowance	$ (0.8)	(10.7)
Equity method investments, reported without a reporting lag	0.029
Equity method investments, reported with a one-moth reporting lag	0.054
Equity Securities at Modified Cost	$ 513.5	586.5	[1]
Convertible Securities at Fair Value	$ 281.2	283.4	[1]
Debt Securities, Available-for-Sale, Unrealized Loss Amount, Average, Percentage	0.088
Alternative Energy Partnership Investments
Debt and Equity Securities, FV-NI [Line Items]
Equity Method Investments	$ 17.3	16.3	[1]
Equity Method Limited Liability Investments
Debt and Equity Securities, FV-NI [Line Items]
Equity Method Investments	221.7	217.0	[1]
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Debt and Equity Securities, FV-NI [Line Items]
Total Assets	5,720.9	5,585.9
Liabilities	2,565.5	2,367.0
Net Loss	244.5	381.8		585.1
Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	0.9	5.8
Other liabilities		25.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	730.3	936.8
Unrealized loss position for 12 months or longer	721.2	308.9
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	237.8
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	92.5
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	64.3
Other Non-Governmental Issuers
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,265.6
Debt Security, Government, Non-US [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	0.6	0.9
Unrealized loss position for 12 months or longer	0.6	0.9
Allowance for credit losses	0.0	0.0
Additions for Securities for which No Previous Expected Credit Losses were Recognized		(0.7)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized		0.0
Write-offs Charged Against Allowance		0.0
Corporate Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	383.8	499.7
Unrealized loss position for 12 months or longer	378.3	125.8
Allowance for credit losses	(7.7)	(8.9)		$ (7.5)
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2.9	6.3
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(1.1)	5.8
Write-offs Charged Against Allowance	(0.4)	(10.7)
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	0.0	35.2
Other liabilities	0.0
Dividends on Equity Securities Excluding Alternative Investments
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	0.1
Investment-grade | Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	704.8	904.0
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	$ 25.5	$ 32.8
Percentage of unrealized loss position to amortized cost basis of available for sale security average		11.00%

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.

[2] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.

[3] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.