CHANGE IN OPERATING WORKING CAPITAL - GTN's Carty Lateral Construction Accrual (Details) - USD ($) $ in Millions	9 Months Ended			12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	[1]	Dec. 31, 2016	Dec. 31, 2015
Non-cash items
Capital expenditures	$ 26	$ 21
GTN
Non-cash items
Accruals for capital expenditures					$ 10
Capital expenditures				$ 10

[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).