Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pre-tax operating results of WM Life Re
Total revenues	$ 87.5	$ 56.8	$ 259.8	$ 163.2
General and administrative expenses	(44.1)	(42.4)	(162.2)	(144.9)
Pre-tax income (loss) from continuing operations	$ 8.4	(13.8)	$ 2.6	(72.8)
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue		0.0		1.2
Change in fair value of variable annuity liability, included in other revenue		0.0		(0.3)
Change in fair value of derivatives, included in other revenue		0.0		(2.0)
Foreign exchange, included in other revenue		0.0		1.4
Total revenues		0.0		0.3
Death benefit claims paid, included in general and administrative expenses		0.0		(0.3)
General and administrative expenses		(0.5)		(2.4)
Pre-tax income (loss) from continuing operations		$ (0.5)		$ (2.4)

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5618551-113959

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The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
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Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 225
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Amount of other increase (decrease) in accrued obligations to policyholders pertaining to insured events for the period.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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This item represents the change in fair value of derivatives due to changes in foreign currency exchange rates, which is included in other revenues.

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Net increase or decrease in the fair value of the variable annuity liabilities included in earnings, from losses associated with chrages in projected surrender assumptions

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