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		9 Months Ended				3 Months Ended	1 Months Ended								1 Months Ended						9 Months Ended	12 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended		9 Months Ended	61 Months Ended
	Dec. 31, 2011 company	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 OneBeacon Ltd.	Dec. 31, 2012 OneBeacon Ltd.	Sep. 30, 2012 White Mountains	Jul. 31, 2012 HG Global	Dec. 31, 2012 HG Global	Sep. 30, 2013 White Mountains Consolidated Limited Partnership	Dec. 31, 2012 White Mountains Consolidated Limited Partnership	Sep. 30, 2013 A W G Dewar Inc	Dec. 31, 2012 A W G Dewar Inc	Sep. 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	Sep. 30, 2013 Preferred Stock HG Global	Dec. 31, 2012 Preferred Stock HG Global	Sep. 30, 2013 Common Stock HG Global	Dec. 31, 2012 Common Stock HG Global	Sep. 30, 2012 Common shares and paid-in surplus	Dec. 31, 2012 Common shares and paid-in surplus HG Global	Sep. 30, 2012 Noncontrolling interest	Sep. 30, 2013 Noncontrolling interest Build America Mutual	Sep. 30, 2013 Noncontrolling interest Build America Mutual	Dec. 31, 2011 Tuckerman Fund I Common Stock	Jul. 31, 2012 BAM Management	Sep. 30, 2013 Interest Rate Cap Sirius International Group Ltd	Jun. 30, 2022 Interest Rate Cap Sirius International Group Ltd
Basis of Presentation
Ownership interest (as a percent)						75.20%	75.20%												97.30%	97.30%	88.70%	88.70%
Surplus Notes									$ 503.0	$ 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						24.80%	24.80%								25.00%				97.30%		88.70%
Funds to Capitalized HG Global								600.0	594.5																				14.5
Fair Value of Net Assets Acquired																							(2.2)	2.2	2.2
Net loss attributable to non-controlling interests		1.1	30.9	12.7	2.0																					(11.4)	(52.4)
Non-cumulative perpetual preference shares issued																	250.0
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
Preferred Stock, Dividend Rate, Percentage																	7.506%
Term of LIBOR loan																	3 months
basis points																	3.20%													3.95%
years of interest rate cap																															5 years
Derivative, Cap Interest Rate																														8.30%
Noncontrolling interest - other											$ 45.1	$ 41.5	$ 2.9	$ 2.8		$ 0.2
Limited Liability Company LLC or Limited Partnership LP, Ownership Interest															100.00%