OneBeacon Pension Plan (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Contribution Plan Disclosure [Line Items]
Other Income	$ 34.8	$ 38.3	$ 110.8	$ 114.4
Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Assets for Plan Benefits	12.6		12.6
Transfer of assets	47.1		47.1
OneBeacon [Member]
Defined Contribution Plan Disclosure [Line Items]
Other Income	1.8	$ 3.9	$ 3.5	$ (0.4)
OneBeacon [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Other Income	$ 3.2