Investments - Allowance for Loan Losses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Collective valuation allowance for losses, beginning of period	$ 1.1	$ 1.2
Addition to (reduction of) allowance for losses	0.1	(0.1)
Collective valuation allowance for losses, end of period	$ 1.2	$ 1.1