Investments - Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 4.3		$ 4.3		$ 4.6
Less: Allowances for losses on impaired loans	0.0		0.0		0.0
Impaired loans, net	4.3		4.3		4.6
Unpaid principal balance of impaired loans	5.8		5.8		$ 6.1
Impaired loans, average investment during the period (amortized cost)	4.4	$ 4.7	4.5	$ 7.7
Interest income recognized on impaired loans, on an accrual basis	0.1	0.1	0.3	0.3
Interest income recognized on impaired loans, on a cash basis	0.1	0.1	0.3	0.3
Interest income recognized on troubled debt restructured loans, on an accrual basis	$ 0.0	$ 0.0	$ 0.0	$ 0.0