CASH DISTRIBUTIONS TO COMMON UNITS - Distributions Paid (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Common units
Distributions
Per Unit Distribution, paid (in dollars per unit)	$ 0.94	$ 0.89	$ 2.72	$ 2.57
Common units and General Partner interest combined
Distributions
Total cash distribution	$ 65	$ 59	$ 184	$ 169
Total distribution of general partner interest and IDR payment	3		7
TC PipeLines GP, Inc. | General Partner
Distributions
Total distribution for General Partner interest	1	1	$ 3	$ 3
General partner interest (as a percent)			2.00%	2.00%
TC PipeLines GP, Inc. | Common units and General Partner interest combined
Distributions
Incentive distribution paid to the General Partner	$ 2	$ 1	$ 4	$ 2