Retirement and Postretirement Plans (Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 119.5		$ 113.0
Service cost	0.8		0.7		0.8
Interest cost	4.2		4.7		5.2
Settlement gain	0		(0.7)
Special termination benefit cost	0.3	[1]	0.6	[1]	0.8	[1]
Assumption changes	(13.0)		11.2
Actuarial gain	(0.5)		(0.2)
Benefits and expenses paid with plan assets	6.2		7.5
Benefits paid directly by OneBeacon	2.2		2.3
Projected benefit obligation at end of year	102.9		119.5		113.0
Change in plan assets:
Fair value of plan assets at beginning of year	124.7		120.8
Actual return on plan assets	24.3		11.4
Benefits and expenses paid with plan assets	6.2		7.5
Fair value of plan assets at end of year	142.8		124.7		120.8
Funded status at end of year	39.9		5.2
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	64.1		32.8
Net liabilities of the Non-qualified Plan recorded in other liabilities	(24.2)		(27.6)
Defined Benefit Plan, Amounts Recognized in Balance Sheet	39.9		5.2
Qualified Plan
Change in plan assets:
Fair value of plan assets at end of year	142.1		124.0
Amounts recognized in the financial statements
Net assets of the Qualified Plan recorded in other assets	64.1
Non-qualified Plan
Amounts recognized in the financial statements
Net liabilities of the Non-qualified Plan recorded in other liabilities	(24.2)
Non-qualified Plan | OneBeacon Ltd.
Change in plan assets:
Amount set aside in irrevocable rabbi trust	$ 16.2

[1] Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.