Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Expense Amortized
Beginning of period	$ 108.1
End of period (4)	$ 141.7	$ 108.1
OneBeacon | Performance Shares | Long Term Incentive Plan
Target options outstanding
Beginning of period	517,470	493,421	563,190
Shares paid or expired (in shares)	(181,290)	(142,138)	(238,658)
New grants	154,887	165,800	179,000
Assumed forfeitures and cancellations (in shares)	(41,632)	(387)	(10,111)
End of period (4)	449,435	517,470	493,421
Expense Amortized
Beginning of period	$ 3.4	$ 4.0	$ 1.2
Payments and deferrals	(1.5)	(1.0)	0.0
Assumed forfeitures and cancellations	(0.1)	0.0	(0.1)
Expense recognized	(0.4)	(0.4)	(2.9)
End of period (4)	$ 1.4	$ 3.4	$ 4.0
Performance Cycle From 2012 to 2014 | Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	45.70%
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage		37.10%