v3.10.0.1

Fair Value Measurements (Reconciliation with Level 3 Inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2017	Oct. 01, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (109.1)	$ (120.8)	$ (120.8)	$ (273.2)
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
Recorded to Income	(28.4)	54.4
Recorded to regulatory assets - gas costs	(10.0)	69.7
Transfers into Level 3	(6.8)	(0.8)
Transfers out of Level 3	9.6	(0.7)
Purchases	2.9	1.0
Settlements	44.4	28.8
Natural Gas Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(94.2)	(114.6)	(114.6)	(264.1)
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
Recorded to Income	(22.4)	62.0
Recorded to regulatory assets - gas costs	(10.0)	69.7
Transfers into Level 3	(6.8)	(0.8)
Transfers out of Level 3	9.6	(0.7)
Settlements	50.0	19.3
Electricity Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(14.9)	(6.2)	(6.2)	(9.1)
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
Recorded to Income	(6.0)	(7.6)
Purchases	2.9	1.0
Settlements	(5.6)	9.5
Washington Gas Light Company \| Natural Gas Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(94.1)	(122.6)	$ (122.6)	$ (251.6)
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
Recorded to Income	(4.5)	44.2
Recorded to regulatory assets - gas costs	(10.0)	69.7
Transfers into Level 3	(6.9)	(0.4)
Transfers out of Level 3	8.9	(0.4)
Settlements	$ 41.0	$ 15.9

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement 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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- Definition

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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Amount of settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative 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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- Definition

Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into level 3 of the fair value hierarchy.

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- Definition

Amount of transfers of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset) out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as 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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This item represents the amount of the total change in unrealized (holding) gains or losses for the period which are included in regulatory assets. Such unrealized (holding) gains or losses relate to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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WGL HOLDINGS INC 10-K