SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 OneBeacon	Dec. 31, 2010 OneBeacon	Dec. 31, 2009 OneBeacon	Dec. 31, 2011 Sirius	Dec. 31, 2010 Sirius	Dec. 31, 2009 Sirius	Dec. 31, 2009 50%-or-less owned property and casualty investees - Delos	Dec. 31, 2011 50%-or-less owned property and casualty investees - Delos	Aug. 03, 2006 50%-or-less owned property and casualty investees - Delos
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS
Deferred acquisition costs	$ 123.5	$ 114.5	$ 215.0	$ 63.5	$ 61.6	$ 57.2	$ 3.9
Reserves for Unpaid Claims and Claims Adjustment Expenses	3,358.6	3,295.5	3,934.8	2,343.7	2,441.3	2,444.4	73.3
Discount, if any, deducted in Column C	271.6	295.9	317.5	12.8	21.1	29.5
Unearned Premiums	528.0	627.5	1,018.3	319.0	311.2	287.5	26.1
Earned Premiums	1,015.5	1,487.7	1,959.5	912.3	847.9	858.8	33.3
Net investment income	71.4	96.6	125.5	89.9	90.5	107.7	1.7
Current Year	583.8	981.4	1,205.3	672.9	588.1	449.1	24.3
Prior Year	(2.9)	(51.2)	(83.4)	(46.9)	(57.1)	(30.4)
Amortization of deferred policy acquisition costs	219.0	320.7	398.3	181.0	167.5	170.9
Paid Claims and Claims Adjustment Expenses	714.8	1,038.8	1,176.1	642.0	437.0	772.9	18.2
Premiums written	$ 1,062.9	$ 1,236.3	$ 1,906.7	$ 915.7	$ 865.8	$ 806.8	$ 31.7
Rate at which reserves are discounted (as a percent)	4.50%	5.00%	5.30%
Ownership interest in property and casualty insurance affiliates (as a percent)								50.00%	18.00%