Represents the sum of the carrying value of all the Assets Held-for-Sale at the balance sheet date.

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The gross gains that make up a cumulative-effect adjustment on retained earnings.

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Amount of the cumulative effect on retained earnings net of related income tax effect.

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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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Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer.

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Percentage of par value of policy reinsured.

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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Jul. 31, 2012
Basis of Presentation
Assets held for sale	$ 58.0	$ 58.1
OneBeacon Ltd.
Basis of Presentation
Cumulative Effect on Retained Earnings, Net of Tax	1.2	0.5
Ownership interest (as a percent)	75.30%
Noncontrolling equity percentage	24.70%	24.70%
HG Global
Basis of Presentation
Surplus Notes				503.0
Percentage of par value of policy reinsured	15.00%
Noncontrolling equity percentage	3.10%	3.10%
Star & Shield Insurance Exchange
Basis of Presentation
Surplus Notes	19.0
Noncontrolling equity percentage	100.00%	100.00%
White Mountains Advisors L L C [Member]
Basis of Presentation
Investment Advisory, Management and Administrative Fees	2.0		4.4
Preferred Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%
Noncontrolling equity percentage	96.90%
Common Stock \| HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%
Noncontrolling equity percentage	88.40%
Wholly Owned Properties [Member] \| OneBeacon Ltd.
Basis of Presentation
Assets held for sale	58.0	58.1
Investment Income [Member] \| OneBeacon Ltd.
Basis of Presentation
Cumulative-Effect Adjustment, Gross Gains	0.4
Tax Expense [Member] \| OneBeacon Ltd.
Basis of Presentation
Cumulative-Effect Adjustment, Gross Gains	$ 0.6