Summary of Significant Accounting Policies Revenue recognition (Details) (White Mountains Advisors LLC, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
White Mountains Advisors LLC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment Advisory, Management and Administrative Fees	$ 18.4	$ 16.6	$ 15.9

Amount of revenue from providing investment advisory, asset management and administrative services, including, but not limited to, distribution and shareholder services, accounting, tax, legal, regulatory filing, share registration, and shareholder correspondence activities.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration