NET INCOME PER COMMON UNIT - General Partner Interest (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
General Partner \| Common Units \| TC Pipelines, LP
PARTNERS' EQUITY
General partner interest (as a percent)	2.00%	2.00%	2.00%

Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

Name:	us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPManagingMemberOrGeneralPartnerOwnershipInterest
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration