PARTNERS' EQUITY - ATM Equity Issuance Program (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
PARTNERS' EQUITY
Net proceeds from issuance of common units	[1]			$ 92,000,000
General Partner
PARTNERS' EQUITY
Net proceeds from issuance of common units				$ 2,000,000
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				1,542,921
Net proceeds from issuance of common units				$ 90,000,000
Sales agent commissions				1
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner
PARTNERS' EQUITY
Equity contribution				$ 2,000,000

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