INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
State income taxes
Total state income taxes	$ 1	$ 1	[1]
Portland Natural Gas Transmission System
Income Taxes
Effective income tax rate (as a percent)	3.80%			3.80%
State income taxes
Current	$ 1	1
Total state income taxes	$ 1	$ 1

[1] Recast to consolidate PNGTS (Refer to Note 2).