SIGNIFICANT ACCOUNTING POLICIES - Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
State income tax benefit (expense)
Total state income taxes	$ 1	$ (1)	$ (1)
PNGTS
Income Taxes
Effective income tax rate (as a percent)	2.60%	3.50%	3.80%
State income tax benefit (expense)
Current	$ 1	$ 2	$ 1
Deferred	(2)	(1)
Total state income taxes	$ (1)	$ 1	$ 1