Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011
Loss Contingencies [Line Items]
Rent expense	$ 15.5	$ 11.8	$ 12.1
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	14.3
Capital lease payments due in two years	13.4
Capital lease payments due in three years	11.4
Capital Lease payments due in four years and thereafter	50.6
Commitments to fund other-long term investments	82.2
Reserve for probable liability from mandatory shared market mechanisms	8.5
OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease payments due next year	1.7
Cash proceeds received a result of entering into the sale-leaseback transactions				$ 23.1
Capital lease obligation	1.7	7.1
Capital lease asset	2.4	$ 7.0
Furniture and equipment | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease term (in years)				5 years
Capitalized software | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease term (in years)				4 years
Furniture, Equipment and Capitalized Software [Member] | OneBeacon
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Capital lease asset	$ 1.3