OneBeacon Pension Plan (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Contribution Plan Disclosure [Line Items]
Other Income	$ 36.2	$ 48.7	$ 76.0	$ 76.1
Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Pension Plan Liabilities, Current			0.3
Defined Benefit Plan, Assets for Plan Benefits	42.4		42.4
Transfer of assets	12.0		12.0
OneBeacon [Member]
Defined Contribution Plan Disclosure [Line Items]
Other Income	0.8	$ (1.2)	$ 1.7	$ (4.3)
OneBeacon [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Other Income	$ 0.1