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			1 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 01, 2012 Installment	May 31, 2011 Chief Executive Officer	May 25, 2011 Chief Executive Officer Installment
Target Performance Share activity
Beginning of period (in shares)	927,000	630,000
Issued (in shares)	0	300,000		630,000
Shares issued during the period related to performance shares earned	(9,000)	0
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	918,000	930,000
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 9.6	$ 7.7
Issued	0	4.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value	0	0
Assumed forfeitures and cancellations	0	0
Expense recognized	(0.8)	(0.4)
Unamortized grant date fair value, ending balance	$ 8.8	$ 11.9
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
Years Performance Shares will be Forfeited				5 years