Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - Jun. 30, 2015 - Restricted Stock Units (RSUs) [Member] - OneBeacon - One Beacon Long Term Incentive Plan - USD ($) $ in Millions	Total	Total
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	209,369	209,369
Expense recognized	$ 0.3	$ 0.4