Derivative Instruments - Gains and Losses on Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Nicor Gas [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Realized gains (losses) related to hedging natural gas costs	$ (10)	$ (18)	$ (12)	$ (21)
Elizabethtown Gas [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Realized gains (losses) related to hedging natural gas costs	$ (4)	$ (4)	$ (10)	$ (8)

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration