CHANGE IN OPERATING WORKING CAPITAL - Certain Non-Cash Items Excluded from Change in Operating Working Capital (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 01, 2016
Non-cash items
Payments of capital expenditures		$ 28	$ 54	$ 10
Accruals for capital expenditures			10
Accrual of costs related to acquisition of 49.9% interest in PNGTS (Note 6)			2
GTN
Non-cash items
Payments of capital expenditures	$ 10
Accruals for capital expenditures			10
Portland Natural Gas Transmission System | Former parent, TransCanada subsidiaries | Transaction between entities under common control
Non-cash items
Accrual of costs related to acquisition of 49.9% interest in PNGTS (Note 6)			$ 2
Interest acquired (as a percent)		49.90%	49.90%		49.90%