Retirement and Postretirement Plans (Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 113.0		$ 110.2
Service cost	0.7		0.8		0.8
Interest cost	4.7		5.2		6.0
Settlement gain	(0.7)		(0.6)
Special termination benefit cost	0.6	[1]	0.8	[1]	1.9	[1]
Assumption changes	11.2		7.7
Actuarial (gain) loss	(0.2)		(0.3)
Benefits and expenses paid with plan assets, net of participant contributions	(7.5)		(8.4)
Benefits paid directly by OneBeacon	(2.3)		(2.4)
Projected benefit obligation at end of year	119.5		113.0		110.2
Change in plan assets
Fair value of plan assets at beginning of year	120.8		133.0
Actual return on plan assets	11.4		(3.8)
Benefits and expenses paid with plan assets, net of participant contributions	(7.5)		(8.4)
Fair value of plan assets at end of year	124.7		120.8		133.0
Defined Benefit Plan, Funded Status of Plan	5.2		7.8
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	32.8		34.3
Net liabilities of the Non-qualified Plan recorded in other liabilities	(27.6)		(26.5)
Defined Benefit Plan, Amounts Recognized in Balance Sheet	5.2		7.8
Qualified Plan
Change in plan assets
Fair value of plan assets at end of year	124.0		120.5
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	32.8
Non-qualified Plan
Amounts recognized in the financial statements
Net liabilities of the Non-qualified Plan recorded in other liabilities	(27.6)
Non-qualified Plan | OneBeacon Ltd.
Change in plan assets
Amount set aside in irrevocable rabbit trust	$ 13.4

[1] Net actuarial losses resulted from a decrease in investment returns on plan assets in the year ended December 31, 2011 and changes in assumptions in estimating the projected benefit obligation in the years ended December 31, 2012 and 2011