Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended				1 Months Ended	3 Months Ended	12 Months Ended								1 Months Ended						12 Months Ended	3 Months Ended	6 Months Ended	1 Months Ended
	Dec. 31, 2011 company	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 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	Jun. 30, 2013 White Mountains Consolidated Limited Partnership	Dec. 31, 2012 White Mountains Consolidated Limited Partnership	Jun. 30, 2013 A W G Dewar Inc	Dec. 31, 2012 A W G Dewar Inc	Jun. 30, 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	Jun. 30, 2013 Preferred Stock HG Global	Dec. 31, 2012 Preferred Stock HG Global	Jun. 30, 2013 Common Stock HG Global	Dec. 31, 2012 Common Stock HG Global	Dec. 31, 2012 Common shares and paid-in surplus HG Global	Jun. 30, 2013 Noncontrolling interest Build America Mutual	Jun. 30, 2013 Noncontrolling 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%																		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%	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		11.1	(12.1)	11.6	(28.9)																				(20.9)	(41.0)
Non-cumulative perpetual preference shares issued																		250
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												$ 45.0	$ 41.5	$ 2.8	$ 2.8	$ 0.1	$ 0.2
Ownership interest (as a percent)																				97.30%	97.30%	88.70%	88.70%