Employee Share-Based Incentive Compensation Plans (OneBeacon Restsricted Shares) (Details) (One Beacon Restricted Stock [Member], USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	May 31, 2011 One Beacon Long Term Incentive Plan [Member]	Mar. 31, 2012 One Beacon Long Term Incentive Plan [Member]	Mar. 01, 2012 One Beacon Long Term Incentive Plan [Member] Installment	May 31, 2011 Chief Executive Officer [Member] One Beacon Long Term Incentive Plan [Member] Year	May 25, 2011 Chief Executive Officer [Member] One Beacon Long Term Incentive Plan [Member] Installment
Target Performance Share activity
Beginning of period (in shares)	930,000	630,000	630,000	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	0	0
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	930,000	630,000	930,000	630,000
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 11.2	$ 8.5	$ 7.7	$ 8.6
Issued	0	0	4.6	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value	0	0	0	0
Assumed forfeitures and cancelations	0	0	0	0
Expense recognized	(0.8)	(0.4)	(1.9)	(0.5)
Unamortized grant date fair value, ending balance	$ 10.4	$ 8.1	$ 10.4	$ 8.1
Number of Installments in which the Shares Vest							2		4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in Grant, Total					35,000			35,000
Years Performance Shares will be Forfeited								5