The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Carrying amount of the equity interests owned by noncontrolling partners of a limited partnership included in the entity's consolidated financial statements.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount of Net Income (Loss) attributable to noncontrolling interest.

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The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share.

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The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

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Fair value of net assets allocated to noncontrolling interests.

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Funds used to capitalized HG Global.

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The issuance costs and commissions related to preference shares.

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Represents the number of manufacturing companies from whom the entity received the shares.

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Represents the number of states in which insurance is directly written to customers.

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Percentage of par value of policy reinsured.

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Proceeds from the issuance of preferred shares.

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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended				9 Months Ended									1 Months Ended				9 Months Ended	3 Months Ended	9 Months Ended
	Dec. 31, 2011 company	Sep. 30, 2012 state	Sep. 30, 2011	Sep. 30, 2012 state	Sep. 30, 2011	Sep. 30, 2012 OneBeacon Ltd.	Dec. 31, 2011 OneBeacon Ltd.	Sep. 30, 2012 White Mountains [Member]	Sep. 30, 2012 HG Global [Member]	Sep. 30, 2012 White Mountains Consolidated Limited Partnership [Member]	Dec. 31, 2011 White Mountains Consolidated Limited Partnership [Member]	Sep. 30, 2012 Hamer and Bri-Mar [Member]	Sep. 30, 2012 A W G Dewar Inc [Member]	Dec. 31, 2011 A W G Dewar Inc [Member]	Sep. 30, 2012 Passage 2 Health Limited [Member]	Dec. 31, 2011 Passage 2 Health Limited [Member]	May 31, 2007 Preferred Stock Sirius Internation Group Ltd [Member]	May 24, 2007 Preferred Stock Sirius Internation Group Ltd [Member]	Sep. 30, 2012 Preferred Stock HG Global [Member]	Sep. 30, 2012 Common Stock HG Global [Member]	Sep. 30, 2012 Non-controlling interest	Sep. 30, 2012 Non-controlling interest Build America Mutual [Member]	Sep. 30, 2012 Non-controlling interest HG Global [Member]
Basis of Presentation
Ownership percentage in outstanding common shares						75.20%	75.50%
Number of small manufacturing companies owned by the Tuckerman Fund I from whom the entity received shares	2
Ownership interest (as a percent)																			97.00%	89.00%
Percentage of par value of policy reinsured		15.00%		15.00%																15.00%
Number of states in which Esurance insurance directly writes insurance		30		30
Noncontrolling equity percentage in OneBeacon Ltd.						24.80%	24.50%
Funds to Capitalized HG Global								$ 594.5															$ 14.5
Fair Value of Net Assets Acquired									2.2												2.2
Net (Income) Loss Attributable to Noncontrolling Interest		30.9	11.0	2.0	(21.2)																	18.0
Non-cumulative perpetual preference shares issued																	250,000,000
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	$ 57.1	$ 60.8		$ 60.8						$ 52.8	$ 54.2	$ 4.5	$ 3.2	$ 2.3	$ 0.3	$ 0.6