Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Changes in derivative instruments
Beginning balance	$ 56.4	$ 69.2
Purchases	0	0
Realized and unrealized gains (losses)	(5.5)	(6.6)
Transfers in	0wtm_DerivativeTransfersInOutNet	0wtm_DerivativeTransfersInOutNet
Sales/settlements	3.9	(27.0)
Ending balance	47.0	89.6
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0wtm_GainLossOnLiabilityForDeathBenefit	0wtm_GainLossOnLiabilityForDeathBenefit
Level 2 Inputs
Changes in derivative instruments
Beginning balance	33.8	4.7
Purchases	0	0
Realized and unrealized gains (losses)	(6.5)	(7.1)
Transfers in	0	0
Sales/settlements	(3.9)	(27.5)
Ending balance	31.2	25.1
Level 1 Inputs
Changes in derivative instruments
Beginning balance	3.7	1.1
Purchases	0	0
Realized and unrealized gains (losses)	1.6	(0.2)
Transfers in	0	0
Sales/settlements	6.5	0.5
Ending balance	(1.2)	0.4
Variable Annuity \| Level 3 Inputs
Changes in derivative instruments
Beginning balance	0.7	(52.8)
Realized and unrealized gains (losses)	0.4	(1.3)
Transfers in	0	0
Sales/settlements	0	0
Ending balance	1.1	(54.1)
Purchases	0	0
Derivative [Member] \| Level 3 Inputs
Changes in derivative instruments
Beginning balance	18.9	63.4
Purchases	0	0
Realized and unrealized gains (losses)	(0.6)	0.7
Transfers in	0	0
Sales/settlements	1.3	0
Ending balance	$ 17.0	$ 64.1

Fair value of the assets less the liabilities of a derivative or group of derivatives.

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

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Derivative Transfers In (Out), Net

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Purchases that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis.

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Sales and settlements that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis.

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers

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Sales or settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Net increase or decrease in the fair value of the variable annuity death benefit liabilities included in earnings.

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