Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) $ in Millions		1 Months Ended	12 Months Ended
	Feb. 24, 2016 shares	May 31, 2011 shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	May 25, 2011 Installment
Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Employee Service Share-based Compensation, Total Compensation Cost Not Yet Recognized | $			$ 3.9
Defined Contribution Plan, Cost Recognized | $			$ 2.1	$ 0.8
Percentage of common shares outstanding			100.00%
Target Performance Share activity
Issued (in shares)			233,461	226,778
Restricted Share Cliff Vesting in January 2018 [Member] | One Beacon Insurance Company | Restricted Stock
Target Performance Share activity
Issued (in shares)				75,950
Restricted Stock | Sirius Group | W T M Incentive Plan
Target Performance Share activity
Beginning of period			9,205	12,477
End of period (3)			5,235	9,205	12,477
Restricted Stock | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
Beginning of period			382,722	612,500	915,000
Issued (in shares)			170,650	75,950	0
Vested (in shares)			(157,500)	(296,000)	(300,000)
Assumed forfeitures and cancellations (in shares)			0	(9,728)	(2,500)
Employee Service Share-based Compensation Nonvested Awards Forfeited in Period Total Compensation Cost Not yet Recognized | $			$ 0.0	$ (0.1)
End of period (3)			395,872	382,722	612,500
Unamortized grant date fair value
Beginning balance | $			$ 2.5	$ 3.5	$ 6.5
Issued | $			2.3	1.1	0.0
Vested | $			0.0		0.0
Expense recognized | $			(2.7)	(2.0)	(3.0)
Ending balance | $			$ 2.1	$ 2.5	$ 3.5
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)	170,650
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)		630,000
Unamortized grant date fair value
Number of installments in which the shares vest | Installment						4
Forfeited shares		35,000
Years performance shares will be forfeited		5 years
Vesting in January 2019 [Member] | Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)	78,150
Vesting in February 2018 [Member] | Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan
Target Performance Share activity
Issued (in shares)	92,500
Restricted Share Cliff Vesting in January 2017 [Member] | Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
End of period (3)			173,042
Restricted Share Cliff Vesting in January 2018 [Member] | Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan
Target Performance Share activity
End of period (3)			214,506