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		6 Months Ended			1 Months Ended		3 Months Ended		6 Months Ended		1 Months Ended
	Mar. 01, 2012 Restricted Stock	Jun. 30, 2014 Restricted Stock	Jun. 30, 2013 Restricted Stock	Jun. 30, 2014 Restricted Stock	Jun. 30, 2013 Restricted Stock	Feb. 28, 2012 Restricted Stock Installment	May 31, 2011 Restricted Stock Chief Executive Officer	May 25, 2011 Restricted Stock Chief Executive Officer Installment	Jun. 30, 2014 Performance Share	Jun. 30, 2013 Performance Share	Jun. 30, 2014 Performance Share	Jun. 30, 2013 Performance Share	May 31, 2011 Performance Share Chief Executive Officer	May 31, 2011 Performance Cycle 2011 to 2013 Performance Share Chief Executive Officer
Target Restricted Share Activity
Beginning of period (in shares)		612,500	918,000	915,000	927,000				512,938	489,867	493,421	563,190
Issued (in shares)	300,000	0	0	0	0		630,000
Vested (in shares)		0	0	(300,000)	(9,000)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value				$ 0
Assumed forfeitures and cancellations		0	0	(2,500)	0				0	0	(4,145)	(13,665)
Ending of period (in shares)		612,500	918,000	612,500	918,000				512,938	489,867	512,938	489,867
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance		5.7	8.8	6.5	9.6
Issued		0	0	0	0
Assumed forfeitures and cancellations		0	0	0	0				0	(0.1)	0	(0.1)
Expense recognized		(0.8)	(0.8)	(1.6)	(1.6)				0.8	0.9	1.4	1.3
Unamortized grant date fair value, ending balance		$ 4.9	$ 8.0	$ 4.9	$ 8.0				$ 4.1		$ 4.1
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
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years							35,000