Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - Restricted Stock Units (RSUs) [Member] - OneBeacon - One Beacon Long Term Incentive Plan $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2015 USD ($) shares	Sep. 30, 2015 USD ($) shares
Share-based compensation arrangement by share-based payment award
Issued (in shares)		214,430
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	204,179	204,179
Expense recognized | $	$ 0.2	$ 0.6