Summary of Significant Accounting Policies (Details 6) (USD $) In Millions, except Share data, unless otherwise specified					1 Months Ended		1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 OneBeacon Ltd.	Dec. 31, 2010 OneBeacon Ltd.	May 31, 2007 Sirius Group Non-cumulative perpetual preference shares director quarter	May 24, 2007 Sirius Group Non-cumulative perpetual preference shares	May 31, 2007 Sirius Group Non-cumulative perpetual preference shares Until June 30, 2017	May 31, 2007 Sirius Group Non-cumulative perpetual preference shares After June 30,2017	Dec. 31, 2011 White Mountains consolidated limited partnerships	Dec. 31, 2010 White Mountains consolidated limited partnerships	Dec. 31, 2011 A.W.G. Dewar Inc	Dec. 31, 2010 A.W.G. Dewar Inc
Noncontrolling Interest
Noncontrolling ownership percentage in OneBeacon Ltd.			24.50%	24.00%
Non-cumulative perpetual preference shares issued (in shares)					250,000,000
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share)						$ 1,000
Proceeds from non-cumulative perpetual preference shares					$ 245.7
Non-cumulative perpetual preference shares issuance cost and commission						4.3
Number of directors that can be elected to Sirius board					2
Period for non-payment of dividend which give right to shareholders to elect directors on board (in quarters)					6
Number of quarterly dividend payments which results in cessation of the right to elect directors					4
Annual rate of dividend on preferred stock (as a percent)							7.506%
Reference rate (LIBOR) used to determine floating annual rate of the dividend on the Sirius Group preference shares								3 month LIBOR
Amount in excess of LIBOR reference rate used to determine floating dividend rate								3.20%
Floating annual dividend rate (if in excess of LIBOR plus 3.20%)								7.506%
Basis points on comparable treasury issue rate (as a percent)					0.45%
Noncontrolling equity interest	$ 57.1	$ 62.8							$ 54.8	$ 59.7	$ 2.3	$ 3.1