PARTNERS' EQUITY - ATM Equity Issuance Program (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
PARTNERS' EQUITY
Net proceeds from issuance of common units	[1]			$ 126
General Partner
PARTNERS' EQUITY
Net proceeds from issuance of common units				$ 2
TC PipeLines GP, Inc. | General Partner
PARTNERS' EQUITY
Ownership interest in the Partnership (as a percent)		2.00%	2.00%	2.00%	2.00%
ATM Equity Issuance Program | Common Units
PARTNERS' EQUITY
Units sold				2,165,162
Net proceeds from issuance of common units				$ 124
Sales agent commissions				1
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner
PARTNERS' EQUITY
Equity contribution				$ 2

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