SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Accident Year	$ 712.9	$ 732.0	$ 622.1
Prior Accident Year	$ (4.0)	$ 92.0	$ 0.0
Rate at which reserves are discounted (as a percent)	2.50%	2.50%	3.50%
OneBeacon
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	$ 100.7	$ 103.2	$ 103.7
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,389.8	1,342.2	1,054.3
Discount, if any, deducted in Column C	1.1	1.0	3.0
Unearned Premiums	560.3	588.3	544.9
Earned Premiums	1,176.2	1,177.1	1,120.4
Net investment income	45.9	43.4	43.0
Current Accident Year	702.5	725.3	622.1
Prior Accident Year	(1.8)	89.8	0.0
Amortization of deferred policy acquisition	(213.8)	(203.3)	(208.9)
Paid Claims and Claims Adjustment Expenses	677.5	608.6	540.7
Premiums written	1,136.6	1,216.9	1,088.6
Other Operations
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	0.0	0.0	0.0
Reserves for Unpaid Claims and Claims Adjustment Expenses	6.0	7.8	0.0
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	2.1	0.8	0.0
Earned Premiums	8.7	6.1	0.0
Net investment income	0.2	0.2	0.0
Current Accident Year	10.4	6.7	0.0
Prior Accident Year	(2.2)	2.2	0.0
Amortization of deferred policy acquisition	0.0	0.0	0.0
Paid Claims and Claims Adjustment Expenses	10.4	14.8	0.0
Premiums written	$ 10.1	$ 5.9	$ 0.0