Investments - Narrative (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Other Receivables	$ 187,900,000	$ 194,300,000
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	707,800,000
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	707,800,000
Other Liabilities	475,900,000	453,700,000
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses	100,000	0
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securities in An Unrealized Loss Position, In a Position of 12 Months or Longer	0	0
Investments in Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Total	48,000,000	19,500,000
12 Months or Longer	19,500,000	13,900,000
Other Receivables	4,900,000	0
Other Liabilities	36,100,000	5,400,000
U.S. Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Total	15,900,000	6,200,000
12 Months or Longer	7,800,000	5,100,000
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	469,400,000
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	10,100,000
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Schedule of Available-for-sale Securities [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	3,600,000
Other Non-Governmental Issuers [Member]
Schedule of Available-for-sale Securities [Line Items]
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	224,700,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other Receivables	0	0
Other Liabilities	0	0
External Credit Rating, Investment Grade [Member] | Investments in Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Total	(40,000,000)	12,600,000
External Credit Rating, Non Investment Grade [Member] | Investments in Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Total	$ (8,000,000)	$ 6,900,000
Percentage of Unrealized Loss Position to Amortized Cost Basis of Available for Sale Security Average	4.00%	4.00%