Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) (OneBeacon, One Beacon Long Term Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended		1 Months Ended
	Feb. 24, 2015	Mar. 01, 2012	Mar. 31, 2015	Mar. 31, 2014	May 31, 2011	Feb. 28, 2012 Installment	May 25, 2011 Installment
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)			612,500	915,000
Issued (in shares)	75,950	300,000	75,950	0
Vested (in shares)			(296,000)	(300,000)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value			$ 0
Assumed forfeitures and cancellations			(1,500)	(2,500)
Ending of period (in shares)			390,950	612,500
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance			3.5	6.5
Issued			1.1	0
Assumed forfeitures and cancellations			0	0
Expense recognized			(0.6)	(0.8)
Unamortized grant date fair value, ending balance			4.0	5.7
Number of Installments in which the Shares Vest						2
Restricted Stock | Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)					630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest							4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years					35,000
Performance Share
Target Restricted Share Activity
Beginning of period (in shares)			517,470	493,421
Assumed forfeitures and cancellations			(28,920)	(4,145)
Ending of period (in shares)			462,147	512,938
Unamortized grant date fair value
Assumed forfeitures and cancellations			0	0
Expense recognized			0.2	0.6
Unamortized grant date fair value, ending balance			$ 3.3
Performance Share | Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares					35,000
Years Performance Shares will be Forfeited					5 years