CHANGE IN OPERATING WORKING CAPITAL - Components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
CHANGE IN OPERATING WORKING CAPITAL
Change in accounts receivable and other	$ 4	$ (4)		$ 6
Change in other current assets	2	(4)		(1)
Change in accounts payable and accrued liabilities	(7)	5		(2)
Change in accounts payable to affiliates	(3)			(15)
Change in state income taxes payable				(5)
Change in accrued interest	2	2		(3)
Change in operating working capital	$ (2)	$ (1)	[1]	$ (20)	[1]

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