Investments - Impaired Loans (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) loan	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) loan	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) loan
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 4.7		$ 4.7		$ 10.7
Less: Allowances for losses on impaired loans	0.0		0.0		0.0
Impaired loans, net	4.7		4.7		10.7
Unpaid principal balance of impaired loans	6.2		6.2		$ 12.2
Impaired loans, average investment during the period (amortized cost)	4.7	$ 20.7	7.7	$ 26.5
Interest income recognized on impaired loans, on an accrual basis	0.1	0.3	0.2	0.7
Interest income recognized on impaired loans, on a cash basis	0.1	0.3	0.2	0.8
Interest income recognized on restructured loans, on an accrual basis	$ 0.0	$ 0.2	$ 0.0	$ 0.5
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