Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified					3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended									1 Months Ended						1 Months Ended				1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 OneBeacon Ltd.	Dec. 31, 2013 OneBeacon Ltd.	Sep. 30, 2014 Build America Mutual	Sep. 30, 2013 Build America Mutual	Sep. 30, 2014 Build America Mutual	Sep. 30, 2013 Build America Mutual	Jul. 31, 2012 HG Global	Sep. 30, 2014 HG Global	Sep. 30, 2013 HG Global	Sep. 30, 2014 HG Global	Sep. 30, 2013 HG Global	Dec. 31, 2013 HG Global	Sep. 30, 2014 Star & Shield Insurance Exchange	Sep. 30, 2014 White Mountains Consolidated Limited Partnership	Dec. 31, 2013 White Mountains Consolidated Limited Partnership	Sep. 30, 2014 A W G Dewar Inc	Dec. 31, 2013 A W G Dewar Inc	Feb. 19, 2014 WOBI	May 31, 2007 Preferred Stock Sirius International Group Ltd	May 24, 2007 Preferred Stock Sirius International Group Ltd	Sep. 30, 2014 Preferred Stock HG Global	Dec. 31, 2013 Preferred Stock HG Global	Sep. 30, 2014 Common Stock HG Global	Dec. 31, 2013 Common Stock HG Global	Jul. 31, 2012 Common shares and paid-in surplus HG Global	Sep. 30, 2014 Non-controlling interest Star & Shield Insurance Exchange	Sep. 30, 2014 Non-controlling interest WOBI	Sep. 30, 2014 Non-controlling interest QuoteLab	Jul. 31, 2012 BAM Management	Sep. 30, 2013 Interest Rate Cap Sirius International Group Ltd	Sep. 30, 2014 Interest Rate Cap Sirius International Group Ltd	Jul. 30, 2013 Interest Rate Cap Sirius International Group Ltd
Basis of Presentation
Ownership interest (as a percent)			75.30%	75.20%																60.70%			96.90%		88.40%
Surplus Notes									$ 503.0						$ 17.0
Partners Capital Contribution									594.5																						14.5
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%
Fair Value of Net Assets Acquired																											2.2
Income (Loss) from Continuing Operations before Income Taxes, Domestic					(13.1)	(15.1)	(29.7)	(60.2)		3.8	10.5	13.4	28.2
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																45.5	46.1
Non-controlling equity interest	$ 488.7	$ 491.9								$ 18.0		$ 18.0		$ 16.6				$ 3.2	$ 3.1									$ (11.1)	$ 3.8	$ 18.0
Term of LIBOR loan																					3 months