Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest
Accumulated other comprehensive income (loss) beginning balance	$ 10.5	$ (21.2)
Increase (decrease) in accumulated other comprehensive income (loss):
Amortization of net actuarial losses recognized during the year	0.3	0.9
Net actuarial (losses) gains occurring during the year	(18.8)	30.8
Accumulated other comprehensive (loss) income ending balance	(8.0)	10.5
Other Postretirement Benefit Plan, Defined Benefit [Member]
Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets
Projected benefit obligation	27.1	24.2
Accumulated benefit obligation	27.1	24.2
Fair value of plan assets	0	0
Qualified Plan [Member]
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets
Projected benefit obligation	92.6	78.7
Accumulated benefit obligation	92.6	78.7
Fair value of plan net assets	146.0	142.8
Scenario, Forecast [Member] | OneBeacon Ltd.
Increase (decrease) in accumulated other comprehensive income (loss):
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost during 2012			$ 1.2