Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) (Restricted Stock, Long Term Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	0 Months Ended	12 Months Ended
	May 31, 2011	Mar. 01, 2012 Installment	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 25, 2011 Installment
One Beacon Insurance Company
Unamortized grant date fair value
Forfeited shares	35,000
One Beacon Insurance Company | Chief Executive Officer
Target Performance Share activity
Issued (in shares)	630,000	300,000
Unamortized grant date fair value
Number of installments in which the shares vest		2				4
Forfeited shares	35,000
Years performance shares will be forfeited	5 years
OneBeacon
Target Performance Share activity
Beginning of period			915,000	927,000	630,000
Issued (in shares)			0	0	300,000
Vested (in shares)			(300,000)	(9,000)	(667)
Assumed forfeitures and cancellations (in shares)			(2,500)	(3,000)	(2,333)
Ending December 31,			612,500	915,000	927,000
Unamortized grant date fair value
Beginning balance			$ 6.5	$ 9.6	$ 7.7
Issued			0	0	4.5
Vested			0		0
Assumed forfeitures and cancellations			0	0	0
Expense recognized			(3.0)	(3.1)	(2.6)
Ending balance			$ 3.5	$ 6.5	$ 9.6