Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss Contingencies [Line Items]
Rent expense	$ 14.1	$ 14.0	$ 12.9
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	11.4
Capital lease payments due in two years	9.5
Capital lease payments due in three years	8.0
Capital Lease payments due in four years and thereafter	32.6
Commitments to fund other-long term investments	138.1
Assets	6,544.7	10,282.6
Reserve for probable liability from mandatory shared market mechanisms	8.0
OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease obligation		1.7
Capital lease asset		$ 2.4
Assets Held under Capital Leases [Member] | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Assets	$ 1.3