Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncement, Early Adoption [Line Items]
Net premiums earned	$ 577.0	$ 584.0	$ 602.9	$ 580.1	$ 622.8	$ 650.7	$ 646.4	$ 568.5	$ 2,344.1	$ 2,488.4	$ 2,182.7
Other income	4.8	1.8	5.2	0.6	1.0	0.7	0.9	0.9	12.4	3.5	2.1
Other expense	1.8	$ 2.2	$ 1.7	$ 1.1	$ 1.9	$ 1.3	$ 1.2	$ 1.8	6.8	6.2	$ 8.6
Variable Interest Entity, Not Primary Beneficiary
New Accounting Pronouncement, Early Adoption [Line Items]
VIE, maximum exposure to loss	$ 996.8								996.8
Derivative
New Accounting Pronouncement, Early Adoption [Line Items]
Net premiums earned									0.1
Other income									7.7
Other expense									$ 3.4	$ 1.0