Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) - OneBeacon [Member] - One Beacon Long Term Incentive Plan $ in Millions			1 Months Ended	3 Months Ended
	Feb. 28, 2017 shares	Feb. 24, 2016 shares	May 31, 2011 shares	Mar. 31, 2017 USD ($) shares	Mar. 31, 2016 USD ($) shares	May 25, 2011 Installment
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)				395,872	382,722
Issued (in shares)	461,160	170,650		461,160	170,650
Vested (in shares)				(157,500)	(157,500)
Assumed forfeitures and cancellations				0	0
Ending of period (in shares)				699,532	395,872
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance | $				$ 2.1	$ 2.5
Issued | $				7.4	2.3
Assumed forfeitures and cancellations | $				0.0	0.0
Expense recognized | $				(0.9)	(0.5)
Unamortized grant date fair value, ending balance | $				$ 8.6	$ 4.3
Restricted Stock | Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)			630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest | Installment						4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years			35,000
Performance Share
Target Restricted Share Activity
Beginning of period (in shares)				452,519	449,435
Assumed forfeitures and cancellations				(13)	(4,079)
Ending of period (in shares)				309,796	441,206
Unamortized grant date fair value
Assumed forfeitures and cancellations | $				$ 0.0	$ 0.0
Expense recognized | $				(0.3)	$ (0.4)
Unamortized grant date fair value, ending balance | $				$ 1.8
Performance Share | Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares			35,000
Years Performance Shares will be Forfeited			5 years
Restricted Stock Units (RSUs) [Member]
Target Restricted Share Activity
Issued (in shares)				240,160	222,449
Ending of period (in shares)				208,499
Unamortized grant date fair value
Expense recognized | $				$ (0.6)	$ (0.4)
Restricted Share Cliff Vesting in August 2018 [Member] | Restricted Stock
Target Restricted Share Activity
Issued (in shares)	235,000
Restricted Share Cliff Vesting in January 2020 [Member] | Restricted Stock
Target Restricted Share Activity
Issued (in shares)	115,450
Restricted Share Vesting in February 2018 and February 2019 [Member] | Restricted Stock
Target Restricted Share Activity
Issued (in shares)	110,710
Restricted Share Vesting in February 2018 [Member] | Restricted Stock
Target Restricted Share Activity
Issued (in shares)		92,500
Restricted Share Vesting in February 2019 [Member] | Restricted Stock
Target Restricted Share Activity
Issued (in shares)		78,150