Investments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	$ 207,300,000	$ 222,400,000
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,545,300,000
Equity securities, FV-NI, unrealized gain (loss)	111,900,000
Debt and equity securities, realized gain (loss), excluding other-than-temporary impairment	4,200,000
Cumulative increases in the carrying value due to observable transactions	0
Cumulative decreases in the carrying value due to observable transactions	0
Debt and equity securities, gain (loss), excluding other-than-temporary impairment loss	10,000,000
Total Assets	14,916,500,000	14,341,900,000
Liabilities	10,908,800,000	9,778,500,000
Net Income (Loss)	(120,500,000)	409,900,000	$ 531,100,000
Outstanding commitments to fund equity method limited liability investments	198,500,000
Allowance for credit losses	(7,500,000)	(3,300,000)	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	5,600,000	7,100,000
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(1,100,000)	(400,000)
Write-offs Charged Against Allowance	(300,000)	(1,400,000)
Alternative Energy Partnership Investments
Debt and Equity Securities, FV-NI [Line Items]
Equity Method Investments	39,600,000	21,300,000
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Debt and Equity Securities, FV-NI [Line Items]
Total Assets	5,042,500,000	3,554,500,000	2,368,100,000
Liabilities	2,074,800,000	1,602,500,000	817,200,000
Net Income (Loss)	585,100,000	74,900,000	78,000,000
Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	600,000	5,100,000
Other liabilities	12,700,000	4,300,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	32,700,000	31,700,000
Unrealized loss position for 12 months or longer	5,900,000	20,000,000.0
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	406,400,000
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	46,800,000
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	32,600,000
Other Non-Governmental Issuers
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,059,500,000
Foreign Governments
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,200,000	1,100,000
Unrealized loss position for 12 months or longer	600,000	1,000,000.0
Allowance for credit losses	0	(300,000)	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	0	1,200,000
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(300,000)	(200,000)
Write-offs Charged Against Allowance	0
Corporate Bonds and Notes
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	16,000,000.0	10,600,000
Unrealized loss position for 12 months or longer	2,900,000	3,100,000
Allowance for credit losses	(7,500,000)	(3,000,000.0)	$ 0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	5,600,000	5,900,000
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(800,000)	(200,000)
Write-offs Charged Against Allowance	(300,000)	(1,400,000)
Investment-grade | Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	23,700,000	8,000,000
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	$ 9,000,000	$ 23,700,000
Percentage of unrealized loss position to amortized cost basis of available for sale security average	4.00%	11.00%