Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Expense Amortized
Accrued expense at the beginning of period	$ 140.0
Accrued expense at the end of the period	$ 83.2		$ 140.0
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 | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	452,519	449,435	449,435
Shares paid or expired (in shares)	(142,710)	(167,300)
New grants (in shares)	0	163,150
Assumed forfeitures and cancellations	(13)	(4,079)
Ending of period (in shares)	309,796	441,206	452,519
Expense Amortized
Accrued expense at the beginning of period	$ 1.6	$ 1.4	$ 1.4
Payments and deferrals	0.0	(0.7)
Assumed forfeitures and cancellations	0.0	0.0
Expense recognized	0.3	0.4
Accrued expense at the end of the period	1.9	$ 1.1	$ 1.6
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 1.8