Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended				1 Months Ended	3 Months Ended	12 Months Ended								1 Months Ended						12 Months Ended	3 Months Ended	1 Months Ended
	Dec. 31, 2011 company	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 OneBeacon Ltd.	Dec. 31, 2012 OneBeacon Ltd.	Jul. 31, 2012 White Mountains	Sep. 30, 2012 White Mountains	Dec. 31, 2012 White Mountains	Dec. 31, 2012 HG Global	Mar. 31, 2013 White Mountains Consolidated Limited Partnership	Dec. 31, 2012 White Mountains Consolidated Limited Partnership	Mar. 31, 2013 A W G Dewar Inc	Dec. 31, 2012 A W G Dewar Inc	Mar. 31, 2013 Passage 2 Health Limited	Dec. 31, 2012 Passage 2 Health Limited	May 31, 2007 Preferred Stock Sirius International Group Ltd	May 24, 2007 Preferred Stock Sirius International Group Ltd	Mar. 31, 2013 Preferred Stock HG Global	Dec. 31, 2012 Preferred Stock HG Global	Mar. 31, 2013 Common Stock HG Global	Dec. 31, 2012 Common Stock HG Global	Dec. 31, 2012 Common shares and paid-in surplus HG Global	Mar. 31, 2013 Non-controlling interest Build America Mutual	Dec. 31, 2011 Tuckerman Fund I Common Stock	Jul. 31, 2012 BAM Management HG Global
Basis of Presentation
Ownership percentage in outstanding common shares				75.20%	75.20%
Funds to Capitalize HG Global							$ 600.0	$ 594.5																	$ 14.5
Surplus Notes									503.0
Percentage of par value of policy reinsured		15.00%																		15.00%
Ownership interest in VIE (as a percent)																								94.00%
Number of small manufacturing companies owned by the Tuckerman Fund I from whom the entity received shares	2
Noncontrolling equity percentage in OneBeacon Ltd.				24.80%
Funds to Capitalized HG Global						594.5
Fair Value of Net Assets Acquired																						2.2
Net loss (income) attributable to non-controlling interests		0.5	(16.8)																				20.1
Non-cumulative perpetual preference shares issued																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
Noncontrolling interest - other										$ 44.1	$ 41.5	$ 2.5	$ 2.8	$ 0.2	$ 0.2
Ownership interest (as a percent)																		97.30%	97.30%	88.70%	88.70%