PARTNERS' EQUITY - ATM Equity Issuance Program (Details) - USD ($)	1 Months Ended	2 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended
	Mar. 31, 2016	May 09, 2016	Mar. 31, 2017	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016
Partners' Equity
Net proceeds from issuance of common units			$ 71,000,000
Common units subject to rescission			64,000,000				$ 83,000,000
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%
ATM Equity Issuance Program | Common units
Partners' Equity
Units sold			1,197,749
Net proceeds from issuance of common units			$ 69,000,000
Sales agent commissions			704,000
Period of time for Section 5 Securities violations to be filed within				1 year
Reclassification of common unit issuance subject to rescission, net (in units)	400,000	1,600,000
Common units subject to rescission							$ 83,000,000
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner
Partners' Equity
Equity contribution			$ 2,000,000