Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	$ 262.6	$ 207.3
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,653.4
Unrealized loss position for 12 months or longer		0.0
Equity securities, FV-NI, unrealized gain (loss)	6.2
Debt and equity securities, gain (loss), excluding other-than-temporary impairment loss	9.6
Total Assets	13,364.0	14,916.5
Liabilities	10,920.8	10,908.8
Net (Loss) Income	(301.2)	(120.5)	$ 409.9
Outstanding commitments to fund equity method limited liability investments	95.9
Allowance for credit losses	(9.6)	(7.5)	(3.3)
Additions for Securities for which No Previous Expected Credit Losses were Recognized	7.0	5.6
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	5.8	(1.1)
Write-offs Charged Against Allowance	$ (10.7)	(0.3)
Equity method investments, reported without a reporting lag	0.021
Equity method investments, reported with a one-moth reporting lag	0.062
Alternative Energy Partnership Investments
Debt and Equity Securities, FV-NI [Line Items]
Equity Method Investments	$ 16.3	39.6
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Debt and Equity Securities, FV-NI [Line Items]
Total Assets	5,585.9	5,042.5	3,554.5
Liabilities	2,367.0	2,074.8	1,602.5
Net (Loss) Income	381.8	585.1	74.9
Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	5.8	0.6
Other liabilities	25.9	12.7
Debt securities, available-for-sale, unrealized loss position, accumulated loss	936.8	32.7
Unrealized loss position for 12 months or longer	308.9	5.9
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	245.6
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	95.5
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	65.9
Other Non-Governmental Issuers
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,246.5
Foreign Governments
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	0.9	1.2
Unrealized loss position for 12 months or longer	0.9	0.6
Allowance for credit losses	0.0	0.0	(0.3)
Additions for Securities for which No Previous Expected Credit Losses were Recognized	(0.7)	0.0
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.0	(0.3)
Write-offs Charged Against Allowance	0.0
Corporate Bonds and Notes
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	499.7	16.0
Unrealized loss position for 12 months or longer	125.8	2.9
Allowance for credit losses	(8.9)	(7.5)	$ (3.0)
Additions for Securities for which No Previous Expected Credit Losses were Recognized	6.3	5.6
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	5.8	(0.8)
Write-offs Charged Against Allowance	(10.7)	(0.3)
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	35.2	1.2
Other liabilities	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	904.0	23.7
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	$ 32.8	$ 9.0
Percentage of unrealized loss position to amortized cost basis of available for sale security average	11.00%	4.00%