Investments - Impaired Loans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($) loan	Dec. 31, 2014 USD ($)
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 4.6	$ 10.7
Less: Allowances for losses on impaired loans	0.0	0.0
Impaired loans, net	4.6	10.7
Unpaid principal balance of impaired loans	$ 6.1	12.2
Amount of loans in arrears 30 days or less		$ 1.0
Number of loans in arrears 30 days or less | loan	0	2
Impaired loans, average investment during the period (amortized cost)	$ 7.7	$ 21.6	$ 37.6
Interest income recognized on impaired loans, on an accrual basis	0.4	1.2	2.2
Interest income recognized on impaired loans, on a cash basis	0.4	1.3	2.1
Interest income recognized on restructured loans, on an accrual basis	$ 0.0	$ 0.8	$ 1.8