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

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- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(1)

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

This element represents purchases, sales, issuances, and settlements (net) which have taken place during the period in relation to assets measured at fair value on a recurring basis using unobservable inputs (Level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(2)

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

This element represents [net] transfers in to and out of assets measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(3)

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents total gains or losses recorded to regulatory assets or liabilities for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value

+ Details

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Namespace Prefix:	wgl
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Fair Value Measurements (Reconciliation with Level 3 Inputs) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2010	Sep. 30, 2009
Realized and unrealized gains (losses)
Recorded to income					$ 4.6
WGL Holdings, Inc.
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
Purchases and settlements, net	(1.7)		5.4		10.5	20.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Value	(11.3)	(27.1)		(17.6)	(11.3)	1.4	(9.6)	(27.6)
Realized and unrealized gains (losses)
Recorded to income	12.7		11.5			1.4
Recorded to regulatory assets, gas costs	1.7		1.1		(13.2)	8.0
Transfers in and/or out of Level 3	3.1		1.0		(3.6)	(1.1)
Balance at end of period			1.4			1.4
Purchases and settlements, net	(1.7)		5.4		10.5	20.7
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Value	(11.3)	(27.1)		(17.6)	(11.3)	1.4	(9.6)	(27.6)
WGL Holdings, Inc. \| Weather Derivative [Member]
Realized and unrealized gains (losses)
Transfers in and/or out of Level 3	3.1				3.6
Washington Gas Light Company
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
Balance at beginning of period	(16.0)		11.6
Purchases and settlements, net	0.8		1.0		0.2	2.9
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Value					(9.0)	16.6	15.2	(6.3)
Realized and unrealized gains (losses)
Recorded to income	1.4		1.9		(6.5)	13.1
Recorded to regulatory assets, gas costs	1.7		1.1		(13.2)	8.0
Transfers in and/or out of Level 3	3.1		1.0		(4.7)	(1.1)
Balance at end of period	(9.0)		16.6		(9.0)	16.6
Purchases and settlements, net	0.8		1.0		0.2	2.9
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Value					$ (9.0)	$ 16.6	$ 15.2	$ (6.3)