Summary of Significant Accounting Policies (Noncontrolling Interest) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended											12 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended		12 Months Ended								61 Months Ended	1 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 White Mountains Consolidated Limited Partnership	Dec. 31, 2012 White Mountains Consolidated Limited Partnership	Dec. 31, 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	Dec. 31, 2013 OneBeacon Ltd.	Dec. 31, 2012 OneBeacon Ltd.	Dec. 31, 2012 White Mountains	Sep. 30, 2012 HG Global	Dec. 31, 2013 HG Global	May 31, 2007 Preferred stocks Sirius Internation Group Ltd Period quarter	May 24, 2007 Preferred stocks Sirius Internation Group Ltd	Dec. 31, 2012 BAM Management	Dec. 31, 2012 Common shares and paid-in surplus	Dec. 31, 2012 Non-controlling Interests	Dec. 31, 2013 BAM Non-controlling Interests		Dec. 31, 2012 BAM Non-controlling Interests		Dec. 31, 2013 Interest Rate Cap Sirius Internation Group Ltd	Jun. 30, 2022 Interest Rate Cap Sirius Internation Group Ltd	May 31, 2007 Interest Rate Cap Preferred stocks Sirius Internation Group Ltd	May 31, 2007 Scenario, 1 [Member] Preferred stocks Sirius Internation Group Ltd	May 31, 2007 Scenario, 1 [Member] Interest Rate Cap Preferred stocks Sirius Internation Group Ltd	May 31, 2007 Scenario, 2 [Member] Preferred stocks Sirius Internation Group Ltd
Noncontrolling Interest [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners																25.00%		24.80%	24.80%
Capital contributions																				$ 594.5	$ 600.0				$ 14.5
Fair Value of Net Assets Acquired																										2.2	(2.2)
Pre-tax loss	134.2	55.3	3.6	151.8	(50.8)	135.1	37.6	140.9	344.9	262.8	98.0																	(78.6)	[1]	(36.3)	[1]
Non-cumulative perpetual preference shares issued																							250,000,000											250,000,000
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share)																								$ 1,000
Percentage of investments recorded at fair value									95.00%
Proceeds from non-cumulative perpetual preference shares																							245.7
Non-cumulative perpetual preference shares issuance cost and commission																							4.3
years of interest rate cap																																5 years	5 years
Derivative, Cap Interest Rate																																8.30%
Preferred Stock Redemption Basis Spread on Treasury Issue Rate																							45.00%
Right to elect directors on board number																							2
Period for non payment of dividend which give right to elect directors on board																							6
Minimum required payment period																							4
Preferred stock dividend rate																						6.00%												7.506%			7.506%
Preference shares reference rate for dividend																																				3 month LIBOR
Preference shares floating rate of dividend amount in excess of libor																																			3.20%
Noncontrolling interest—consolidated limited partnerships												$ 46.1	$ 41.5	$ 3.1	$ 2.8		$ 0.2

[1] BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services.