Investments - Narrative (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	$ 222,400,000				$ 219,700,000				$ 222,400,000	$ 219,700,000
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,424,000,000.0								1,424,000,000.0
Debt securities, available-for-sale, continuous unrealized loss position, less than twelve months, securities that have had recognized credit losses in earnings, aggregate losses					300,000					300,000
Available-for-sale-securities, continuous unrealized loss position, less than twelve months, securities that have had recognized credit losses in earnings, aggregate losses					0					0
Equity securities, FV-NI, unrealized gain (loss)									136,600,000
Decrease in carrying value due to observable transactions									500,000
Debt and equity securities, realized gain (loss), excluding other-than-temporary impairment									2,900,000
Cumulative increases in the carrying value due to observable transactions	0								0
Cumulative decreases in the carrying value due to observable transactions	0								0
Debt and equity securities, gain (loss), excluding other-than-temporary impairment loss	5,800,000								5,800,000
Total Assets	14,341,900,000				12,989,100,000				14,341,900,000	12,989,100,000
Liabilities	9,778,500,000				9,016,800,000				9,778,500,000	9,016,800,000
Net Income	97,500,000	$ 122,300,000	$ 126,100,000	$ 64,000,000.0	124,700,000	$ 129,000,000.0	$ 122,100,000	$ 155,300,000	409,900,000	531,100,000	$ 190,100,000
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings	225,300,000				220,400,000				225,300,000	220,400,000
Outstanding commitments to fund equity method limited liability investments	172,800,000								172,800,000
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Debt and Equity Securities, FV-NI [Line Items]
Total Assets	3,554,500,000				2,368,100,000				3,554,500,000	2,368,100,000	2,805,300,000
Liabilities	1,602,500,000				817,200,000				1,602,500,000	817,200,000	1,030,700,000
Net Income									74,900,000	78,000,000.0	$ 130,400,000
Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	5,100,000				1,000,000.0				5,100,000	1,000,000.0
Other liabilities	4,300,000				19,500,000				4,300,000	19,500,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	31,700,000				20,800,000				31,700,000	20,800,000
Unrealized loss position for 12 months or longer	20,000,000.0				13,300,000				20,000,000.0	13,300,000
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	413,800,000								413,800,000
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	5,200,000								5,200,000
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	12,000,000.0								12,000,000.0
Other Non-Governmental Issuers
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	993,000,000.0								993,000,000.0
Investment-grade | Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	8,000,000.0				9,100,000				8,000,000.0	9,100,000
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	$ 23,700,000				$ 11,700,000				$ 23,700,000	$ 11,700,000
Percentage of unrealized loss position to amortized cost basis of available for sale security average	11.00%				5.00%				11.00%	5.00%