Regulatory Matters (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory assets in other long-term assets (1) (2)	[1],[2]	$ 105	$ 103
Regulatory liabilities		(734)	(669)
Net Regulatory Assets (Liabilities)		(629)	$ (566)
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		$ 5

[1] 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 $5 million as of December 31, 2015 were not earning a return.

[2] Regulatory assets that are not earning a return were not material at December 31, 2015 or 2014.