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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended					3 Months Ended		1 Months Ended	3 Months Ended												1 Months Ended						3 Months Ended		1 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2014 OneBeacon Ltd.	Dec. 31, 2013 OneBeacon Ltd.	Mar. 31, 2014 Build America Mutual	Mar. 31, 2013 Build America Mutual	Jul. 31, 2012 HG Global	Mar. 31, 2014 HG Global	Mar. 31, 2013 HG Global	Dec. 31, 2013 HG Global	Mar. 31, 2014 Star & Shield Insurance Exchange [Member]	Mar. 31, 2014 White Mountains Consolidated Limited Partnership	Dec. 31, 2013 White Mountains Consolidated Limited Partnership	Mar. 31, 2014 A W G Dewar Inc	Dec. 31, 2013 A W G Dewar Inc	Mar. 31, 2014 WOBI [Member]	Feb. 19, 2014 WOBI [Member]	Mar. 31, 2014 QuoteLab [Member]	Mar. 14, 2014 QuoteLab [Member]	May 31, 2007 Preferred Stock Sirius International Group Ltd	May 24, 2007 Preferred Stock Sirius International Group Ltd	Mar. 31, 2014 Preferred Stock HG Global	Dec. 31, 2013 Preferred Stock HG Global	Mar. 31, 2014 Common Stock HG Global	Dec. 31, 2013 Common Stock HG Global	Mar. 31, 2013 Common shares and paid-in surplus HG Global	Mar. 31, 2014 Noncontrolling interest Star & Shield Insurance Exchange [Member]	Jul. 31, 2012 BAM Management	Mar. 31, 2014 Interest Rate Cap Sirius International Group Ltd
Basis of Presentation
Ownership interest (as a percent)				75.30%	75.20%													60.70%		54.30%			96.90%		88.40%
Surplus Notes								$ 503.0	$ 503.0			$ 12.0
Percentage of par value of policy reinsured	15.00%								15.00%
Noncontrolling equity percentage				24.70%	24.80%																		96.90%	97.30%	88.40%	88.70%
Partners Capital Contribution								594.5																					14.5
Fair Value of Net Assets Acquired																											2.2
Income (Loss) from Continuing Operations before Income Taxes, Domestic						(8.6)	(18.4)		4.6	9.9
Net loss attributable to non-controlling interests	0.5	0.5																										4.7
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%
basis points																					3.20%									3.95%
years of interest rate cap																														5 years
Derivative, Cap Interest Rate																														8.30%
Noncontrolling interest - other													46.7	46.1
Non-controlling equity interest	$ 513.4		$ 491.9						$ 18.4		$ 16.6				$ 2.7	$ 3.1	$ 5.0		$ 18.8
Term of LIBOR loan																					3 months

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