Investments - Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 20.7		$ 20.7		$ 32.4
Less: Allowances for losses on impaired loans	0.0		0.0		0.0
Impaired loans, net	20.7		20.7		32.4
Unpaid principal balance of impaired loans	22.2		22.2		$ 33.9
Impaired loans, average investment during the period (amortized cost)	20.7	$ 40.0	26.5	$ 40.0
Interest income recognized on impaired loans, on an accrual basis	0.3	0.6	0.7	1.2
Interest income recognized on impaired loans, on a cash basis	0.3	0.6	0.8	1.0
Interest income recognized on restructured loans, on an accrual basis	$ 0.2	$ 0.5	$ 0.5	$ 1.0