Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) - OneBeacon - One Beacon Long Term Incentive Plan $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 24, 2015 shares	Mar. 01, 2012 shares	May. 31, 2011 shares	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($) shares	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($) shares	Dec. 31, 2014 USD ($) shares	Feb. 28, 2012 Installment	May. 25, 2011 Installment
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)				390,950	612,500	612,500	915,000	915,000
Issued (in shares)	75,950	300,000		0	0	75,950	0
Vested (in shares)				0	0	(296,000)	(300,000)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value | $						$ 0.0
Assumed forfeitures and cancellations				0	0	(1,500)	(2,500)
Ending of period (in shares)				390,950	612,500	390,950	612,500	612,500
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance | $				$ 3.5	$ 4.9	$ 3.5	$ 6.5	$ 6.5
Issued | $				0.0	0.0	1.1	0.0
Assumed forfeitures and cancellations | $				0.0	0.0	0.0	0.0
Expense recognized | $				(0.5)	(0.7)	(1.6)	(2.3)
Unamortized grant date fair value, ending balance | $				$ 3.0	$ 4.2	$ 3.0	$ 4.2	$ 3.5
Number of Installments in which the Shares Vest | Installment									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 | Installment										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)				462,147	512,938	517,470	493,421	493,421
Assumed forfeitures and cancellations				0	0	(28,920)	(4,145)
Ending of period (in shares)				462,147	512,938	462,147	512,938	517,470
Unamortized grant date fair value
Assumed forfeitures and cancellations | $				$ 0.0	$ 0.0	$ 0.0	$ 0.0
Expense recognized | $				(0.3)	$ 1.1	0.2	$ 2.5
Unamortized grant date fair value, ending balance | $				$ 2.0		$ 2.0
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