SIGNIFICANT ACCOUNTING POLICIES - Acquisitions and Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Acquisitions and Goodwill
Goodwill	$ 130	$ 130	[1]
Impairment of goodwill	0
North Baja Pipeline, LLC
Acquisitions and Goodwill
Goodwill	48	48
Tuscarora Gas Transmission Company
Acquisitions and Goodwill
Goodwill	$ 82	$ 82

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