Regulatory Accounting (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Schedule Of Regulatory Assets And Liabilities [Line Items]
Current regulatory assets (1)	[1]	$ 130	$ 70
Non-current regulatory assets		181	125
Total regulatory assets		311	195
Current regulatory liabilities (5)	[2]	2	11
Non-current regulatory liabilities		1,201	769
Total regulatory liabilities		1,203	780
Total regulatory assets and liabilities, net		(892)	(585)
Public Utilities, Property, Plant and Equipment [Abstract]
Property, plant and equipment		6,888	6,351
Property, Plant and Equipment, Net		4,852	4,569
Net Regulatory Assets [Abstract]
Non-current regulatory assets		181	125
Other Regulatory Assets (Liabilities) [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory assets	[3]	160	125
Net Regulatory Assets [Abstract]
Non-current regulatory assets	[3]	160	125
Excess deferred income taxes [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory assets	[4]	15	0
Net Regulatory Assets [Abstract]
Non-current regulatory assets	[4]	15	0
Pension and Other Postretirement Plans Costs [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Remaining amounts of regulatory assets for which no return on investment during recovery period is provided		7	6
Removal Costs [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory liabilities		593	665
Excess deferred income taxes [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory liabilities	[4]	492	0
Other Regulatory Assets (Liabilities) [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory liabilities		116	104
Hurricane Harvey [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory assets	[5]	6	0
Estimated storm restoration costs		25
Estimated storm restoration costs covered by insurance		19
Public Utilities, Property, Plant and Equipment [Abstract]
Property, plant and equipment		5
Property, Plant and Equipment, Net		0
Net Regulatory Assets [Abstract]
Operation and maintenance expense		10
Non-current regulatory assets	[5]	6	$ 0
Property, Plant and Equipment [Member] | Hurricane Harvey [Member]
Public Utilities, Property, Plant and Equipment [Abstract]
Insurance receivable		(5)
Net Regulatory Assets [Abstract]
Insurance receivable		(5)
Regulatory Asset [Member] | Hurricane Harvey [Member]
Public Utilities, Property, Plant and Equipment [Abstract]
Insurance receivable		(4)
Net Regulatory Assets [Abstract]
Insurance receivable		(4)
Other Regulatory Assets (Liabilities) [Member] | Hurricane Harvey [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Non-current regulatory assets		6
Net Regulatory Assets [Abstract]
Non-current regulatory assets		$ 6

[1] Current regulatory assets are included in Other current assets in CERC’s Consolidated Balance Sheets.

[2] Current regulatory liabilities are included in Other current liabilities in CERC’s Consolidated Balance Sheets.

[3] NGD’s actuarially determined pension and other postemployment expense in excess of the amount being recovered through rates is being deferred for rate making purposes. Deferred pension and other postemployment expenses of $7 million and $6 million as of December 31, 2017 and 2016, respectively, were not earning a return.

[4] EDIT will be recovered or refunded to customers as required by tax and regulatory authorities. See Note 13 for additional information.

[5] CERC is not earning a return on its Hurricane Harvey restoration costs.