SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Current Accident Year	$ 649.2	$ 712.9	$ 732.0
Prior Accident Year	$ 14.8	$ (4.0)	$ 92.0
Rate at which reserves are discounted (as a percent)	2.50%	2.50%	2.50%
OneBeacon
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	$ 96.3	$ 100.7	$ 103.2
Reserves for Unpaid Claims and Claims Adjustment Expenses	1,365.6	1,389.8	1,342.2
Discount, if any, deducted in Column C	1.6	1.1	1.0
Unearned Premiums	575.1	560.3	588.3
Earned Premiums	1,100.6	1,176.2	1,177.1
Net investment income	50.6	45.9	43.4
Current Accident Year	640.6	702.5	725.3
Prior Accident Year	15.4	(1.8)	89.8
Amortization of deferred policy acquisition	(206.0)	(213.8)	(203.3)
Paid Claims and Claims Adjustment Expenses	667.1	677.5	608.6
Premiums written	1,100.7	1,136.6	1,216.9
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	5.0	6.0	7.8
Discount, if any, deducted in Column C	0.0	0.0	0.0
Unearned Premiums	1.2	2.1	0.8
Earned Premiums	7.5	8.7	6.1
Net investment income	0.2	0.2	0.2
Current Accident Year	8.6	10.4	6.7
Prior Accident Year	(0.6)	(2.2)	2.2
Amortization of deferred policy acquisition	0.0	0.0	0.0
Paid Claims and Claims Adjustment Expenses	8.8	10.4	14.8
Premiums written	$ 6.5	$ 10.1	$ 5.9