Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability.

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No definition available.

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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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Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 JPY (¥)
Changes in derivative instruments
Collective account values of underlying variable annuities as a percentage of the guarantee value					113.00%	113.00%
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Notional amount					$ 400.0	¥ 50.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	$ 0.0		$ 0.0
Changes in derivative instruments
Beginning of period	13.1	$ 47.0	20.1	$ 56.4
Purchases		0.0		0.0
Realized and unrealized gains (losses)	(0.3)	(2.3)	(2.0)	(7.8)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(12.8)	(9.0)	(18.1)	(12.9)
End of period	0.0	35.7	0.0	35.7
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0.0	0.0	0.0	0.0
Collective account values of underlying variable annuities as a percentage of the guarantee value					109.00%	109.00%
White Mountains Life Re \| Level 2 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning of period	10.3	31.2	16.5	33.8
Purchases		0.0		0.0
Realized and unrealized gains (losses)	(1.8)	0.6	(0.7)	(5.9)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(8.5)	0.7	(15.8)	4.6
End of period	0.0	32.5	0.0	32.5
White Mountains Life Re \| Level 1 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning of period	0.2	(1.2)	0.9	3.7
Purchases		0.0		0.0
Realized and unrealized gains (losses)	(0.2)	5.4	(4.2)	7.1
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements		(9.3)	3.3	(15.9)
End of period	0.0	(5.1)	0.0	(5.1)
Variable Annuity \| White Mountains Life Re \| Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning balance	(0.1)	(1.1)	(0.3)	(0.7)
Realized and unrealized gains (losses)	0.1	0.7	(0.3)	1.1
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	0.0	0.0	0.0	0.0
Ending balance	0.0	(1.8)	0.0	(1.8)
Purchases		0.0		0.0
Derivative [Member] \| White Mountains Life Re \| Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning of period	2.6	17.0	2.7	18.9
Purchases		0.0		0.0
Realized and unrealized gains (losses)	1.7	(8.3)	2.9	(9.0)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(4.3)	(0.4)	(5.6)	(1.6)
End of period	$ 0.0	$ 8.3	$ 0.0	$ 8.3