Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 7.3
Expense Amortized
Accrued expense at the beginning of period	141.7
Accrued expense at the end of the period	$ 114.2		$ 141.7
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period		3 years
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015
Expense Amortized
Percentage of target payout - minimum actual	24.30%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual		45.70%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	449,435	517,470	517,470
Shares paid or expired (in shares)	(167,300)	(181,290)
New grants (in shares)	163,150	154,887
Assumed forfeitures and cancellations	(4,079)	(28,920)
Ending of period (in shares)	441,206	462,147	449,435
Expense Amortized
Accrued expense at the beginning of period	$ 1.4	$ 3.4	$ 3.4
Payments and deferrals	(0.7)	(1.5)
Assumed forfeitures and cancellations	0.0	0.0
Expense recognized	0.4	0.2
Accrued expense at the end of the period	1.1	$ 2.1	$ 1.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 3.1