Retirement and Postretirement Plans (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 110.2	$ 117.1
Service cost	0.8	0.8	0.5
Interest cost	5.2	6.0	6.5
Settlement gain	(0.6)	(2.2)
Special termination benefits expense	0.8	1.9	1.2
Assumption changes	7.7	6.6
Actuarial (gain) loss	(0.3)	(1.5)
Benefits and expenses paid with plan assets, net of participant contributions	(8.4)	(15.9)
Benefits paid directly by OneBeacon	(2.4)	(2.6)
Projected benefit obligation at end of year	113.0	110.2	117.1
Change in plan assets:
Fair value of plan assets at beginning of year	133.0	130.2
Actual return on plan assets	(3.8)	18.7
Benefits and expenses paid with plan assets, net of participant contributions	(8.4)	(15.9)
Fair value of plan assets at end of year	120.8	133.0	130.2
Funded status at end of year	7.8	22.8
Amounts recognized in the financial statements
Net balance recorded in other assets	34.3	50.3
Net balance recorded in other liabilities	(26.5)	(27.5)
Net amount accrued in the financial statements	7.8	22.8
Qualified Plan
Change in plan assets:
Fair value of plan assets at end of year	120.5	132.7
Funded status at end of year	34.3
OneBeacon segment
Change in plan assets:
Funded status at end of year	26.5
Amount set aside under irrevocable rabbi trust for the benefit of Plan participants	$ 12.2