Investments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	$ 219,700,000	$ 245,400,000
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,302,300,000
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)	0
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)	126,000,000.0
Decrease in carrying value due to observable transactions	500,000
Debt and equity securities, realized gain (loss), excluding other-than-temporary impairment	100,000
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	5,000,000.0
Equity method investment, summarized financial information, assets	2,368,100,000	2,805,300,000	$ 2,393,700,000
Equity method investment, summarized financial information, liabilities	817,200,000	1,030,700,000	899,700,000
Equity method investment, summarized financial information, net income (loss)	78,000,000.0	130,400,000	$ 209,300,000
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings	220,400,000	187,000,000.0
Outstanding commitments to fund equity method limited liability investments	97,200,000
Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Other receivables	1,000,000.0	500,000
Other liabilities	19,500,000	10,500,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss	20,800,000	86,600,000
Unrealized loss position for 12 months or longer	13,300,000	22,000,000.0
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	575,900,000
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	7,100,000
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	12,200,000
Other Non-Governmental Issuers
Debt and Equity Securities, FV-NI [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	707,100,000
Investment-grade | Investments in Fixed Maturities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	9,100,000	69,500,000
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	$ 11,700,000	$ 17,100,000
Percentage of unrealized loss position to amortized cost basis of available for sale security average	5.00%	5.00%