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Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) $ in Millions		1 Months Ended	12 Months Ended
	Mar. 01, 2012 Installment shares	May. 31, 2011 shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares	May. 25, 2011 Installment
Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)			226,778
End of period (4)			200,194
Unamortized grant date fair value
Expense recognized | $			$ (0.8)
Restricted Share Cliff Vesting in January 2018 [Member] | One Beacon Insurance Company | Restricted Stock
Target Performance Share activity
Issued (in shares)			75,950
Unamortized grant date fair value
Issued | $			$ 1.1
Restricted Stock | Sirius Group | W T M Incentive Plan
Target Performance Share activity
Beginning of period			3,265	3,789
End of period (4)			3,571	3,265	3,789
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan
Unamortized grant date fair value
Forfeited shares		35,000
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan | Chief Executive Officer
Target Performance Share activity
Issued (in shares)	300,000	630,000
Unamortized grant date fair value
Number of installments in which the shares vest | Installment	2					4
Forfeited shares		35,000
Years performance shares will be forfeited		5 years
Restricted Stock | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
Beginning of period			612,500	915,000	927,000
Issued (in shares)			75,950	0	0
Vested (in shares)			(296,000)	(300,000)	(9,000)
Assumed forfeitures and cancellations (in shares)			(9,728)	(2,500)	(3,000)
End of period (4)			382,722	612,500	915,000
Unamortized grant date fair value
Beginning balance | $			$ 3.5	$ 6.5	$ 9.6
Issued | $			1.1	0.0	0.0
Vested | $			0.0		0.0
Assumed forfeitures and cancellations | $			(0.1)	0.0	0.0
Expense recognized | $			(2.0)	(3.0)	(3.1)
Ending balance | $			$ 2.5	$ 3.5	$ 6.5

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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