Investments - Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 4.4		$ 4.4		$ 4.6
Less: Allowances for losses on impaired loans	0.0		0.0		0.0
Impaired loans, net	4.4		4.4		4.6
Unpaid principal balance of impaired loans	5.9		5.9		$ 6.1
Impaired loans, average investment during the period (amortized cost)	4.5	$ 4.7	4.5	$ 7.7
Interest income recognized on impaired loans, on an accrual basis	0.1	0.1	0.2	0.2
Interest income recognized on impaired loans, on a cash basis	0.1	0.1	0.2	0.2
Interest income recognized on troubled debt restructured loans, on an accrual basis	$ 0.0	$ 0.0	$ 0.0	$ 0.0