INCOME TAXES (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
State income taxes
Deferred	[1]				$ 4
Total state income taxes		$ 1	$ 1	[1],[2]	$ 2	[1]
Portland Natural Gas Transmission System
Income Taxes
Effective income tax rate (as a percent)		3.80%	3.80%		3.80%
State income taxes
Current		$ 1	$ 1		$ (2)
Deferred					4
Total state income taxes		$ 1	$ 1		$ 2

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).

[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).