Summary of Significant Accounting Policies (Noncontrolling Interest) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended											9 Months Ended	12 Months Ended		1 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 White Mountains Consolidated Limited Partnership	Dec. 31, 2011 White Mountains Consolidated Limited Partnership	Dec. 31, 2012 A W G Dewar Inc	Dec. 31, 2011 A W G Dewar Inc	Dec. 31, 2012 Passage 2 Health Limited	Dec. 31, 2011 Passage 2 Health Limited	Dec. 31, 2012 OneBeacon Ltd.	Dec. 31, 2011 OneBeacon Ltd.	Sep. 30, 2012 White Mountains	Dec. 31, 2012 White Mountains	Dec. 31, 2012 HG Global	May 31, 2007 Preferred stocks Sirius Internation Group Ltd quarter	May 24, 2007 Preferred stocks Sirius Internation Group Ltd	Jul. 31, 2012 BAM Management HG Global	Dec. 31, 2012 Non-controlling Interests HG Global	Dec. 31, 2012 BAM Non-controlling Interests Build America Mutual
Noncontrolling Interest [Line Items]
Pre-tax income (loss)	$ (50.8)	$ 135.1	$ 37.6	$ 140.9	$ 98.4	$ 1.2	$ 35.8	$ (37.4)	$ 262.8	$ 98.0	$ 189.3																$ (36.3)
Noncontrolling equity percentage in OneBeacon Ltd.																		24.80%	24.50%
Non-cumulative perpetual preference shares issued																							250,000,000
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share)																								$ 1,000
Preferred Stock Redemption Basis Spread on Treasury Issue Rate																							45.00%
Proceeds from non-cumulative perpetual preference shares																							245.7
Non-cumulative perpetual preference shares issuance cost and commission																								4.3
Right to elect eirectors on board number																							2
Period for non payment of dividend which give right to elect directors on board																							6
Minimum required payment period																							4
Preferred stock dividend rate																						6.00%	7.506%
Preference shares reference rate for dividend																							3 month LIBOR
Floating annual dividend rate fixed																							7.506%
Preference shares floating rate of dividend amount in excess of libor																							3.20%
Noncontrolling interest—consolidated limited partnerships												41.5	54.2	2.8	2.3	0.2	0.6
Capital contributions																				594.5
Funds to capitalize HG Global																				600.0	594.5				14.5
Fair Value of Net Assets Acquired																										$ (2.2)