Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - Restricted Stock Units (RSUs) [Member] - OneBeacon [Member] - One Beacon Long Term Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based compensation arrangement by share-based payment award
Issued (in shares)			233,461	215,698
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	219,112	177,409	219,112	177,409
Expense recognized	$ 0.5	$ 0.2	$ 1.4	$ 0.6