Investments - Impaired Loans (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) loan	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) loan	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) loan
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 4.6		$ 4.6		$ 10.7
Less: Allowances for losses on impaired loans	0.0		0.0		0.0
Impaired loans, net	4.6		4.6		10.7
Unpaid principal balance of impaired loans	6.1		6.1		$ 12.2
Impaired loans, average investment during the period (amortized cost)	4.7	$ 16.8	7.7	$ 22.6
Interest income recognized on impaired loans, on an accrual basis	0.1	0.2	0.3	1.0
Interest income recognized on impaired loans, on a cash basis	0.1	0.3	0.3	1.1
Interest income recognized on restructured loans, on an accrual basis	$ 0.0	$ 0.2	$ 0.0	$ 0.7
Number of loans in arrears 30 days or less | loan	0		0		2
Amortized cost of loans in arrears 30 days or less					$ 1.0