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Derivatives (Realized and Unrealized Gains (Losses)) (Details) - White Mountains Life Re $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ (2.0)
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	1.8
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(4.8)
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ 1.0

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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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- References

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