Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) (One Beacon Long Term Incentive Plan, One Beacon Restricted Stock, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		4 Months Ended	6 Months Ended			1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Feb. 28, 2012 Installment	May 31, 2011 Chief Executive Officer	May 25, 2011 Chief Executive Officer Installment
Target Performance Share activity
Beginning of period (in shares)	918,000	930,000		927,000	630,000
Issued (in shares)	0	0	300,000	0	300,000		630,000
Shares issued during the period related to performance shares earned	0	0		(9,000)	0
Assumed forfeitures and cancellations	0	0		0	0
Ending of period (in shares)	918,000	930,000	930,000	918,000	930,000
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 8.8	$ 11.9		$ 9.6	$ 7.7
Issued	0	0		0	4.6
Vested	0	0		0	0
Assumed forfeitures and cancellations	0	0		0	0
Expense recognized	(0.8)	(0.7)		(1.6)	(1.1)
Unamortized grant date fair value, ending balance	$ 8.0	$ 11.2	$ 11.2	$ 8.0	$ 11.2
Number of Installments in which the Shares Vest						2		4
Annual reduction in performance shares							35,000
Years Performance Shares will be Forfeited							5 years