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
	Mar. 31, 2016	Mar. 31, 2015
Share-based compensation arrangement by share-based payment award
Issued (in shares)	222,449	207,848
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	198,928
Expense recognized	$ 0.4	$ 0.1