Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Rent expense	$ 3.5	$ 3.4	$ 2.0
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	2.7
Capital lease payments due in two years	2.4
Capital lease payments due in three years	2.2
Capital Lease payments due in four years and thereafter	9.1
Commitments to fund other-long term investments	109.3
Swedish Tax Authority | Sirius Group
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Estimate of possible loss (up to)	$ 19.0