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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
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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
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of purchases 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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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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-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Derivative Transfers In (Out), Net

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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.

+ References

No definition available.

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Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in derivative instruments
Purchases	$ 1,394.3	$ 3,332.5
Level 2
Changes in derivative instruments
Purchases	783.8	2,007.9
Level 1
Changes in derivative instruments
Purchases	$ 514.7	1,209.3
White Mountains Life Re
Changes in derivative instruments
Realized and unrealized gains (losses)			$ (2.0)
Variable Annuity Liabilities \| White Mountains Life Re \| Level 3
Changes in variable annuity (liabilities)
Beginning balance		0.0	0.3
Purchases			0.0
Realized and unrealized (losses) gains			(0.3)
Transfers in			0.0
Sales/settlements			0.0
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re
Changes in derivative instruments
Beginning of period		0.0	20.1
Purchases			0.0
Realized and unrealized gains (losses)			2.0
Transfers in			0.0
Sales/settlements			(18.1)
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re \| Level 3
Changes in derivative instruments
Beginning of period		0.0	2.7
Purchases			0.0
Realized and unrealized gains (losses)			(2.9)
Transfers in			0.0
Sales/settlements			(5.6)
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re \| Level 2
Changes in derivative instruments
Beginning of period		0.0	16.5
Purchases			0.0
Realized and unrealized gains (losses)			0.7
Transfers in			0.0
Sales/settlements			(15.8)
Ending balance			0.0
Derivative Instruments \| White Mountains Life Re \| Level 1
Changes in derivative instruments
Beginning of period		$ 0.0	0.9
Purchases			0.0
Realized and unrealized gains (losses)			4.2
Transfers in			0.0
Sales/settlements			3.3
Ending balance			$ 0.0