NET INCOME PER COMMON UNIT - General Partner Effective Interest and Allocated Incentive Distributions (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Partners' Equity
Incentive distributions allocated to the General Partner	$ 2	$ 1	$ 3	$ 1
TC PipeLines GP, Inc. \| General Partner
Partners' Equity
General partner interest (as a percent)	2.00%	2.00%	2.00%	2.00%

Amount of cash or stock or unit payment to a limited liability corporation managing member or limited partnership general partner after the end of the accounting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 73
-URI http://asc.fasb.org/extlink&oid=6926462&loc=SL5163674-159010

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 72
-URI http://asc.fasb.org/extlink&oid=6926462&loc=SL5163672-159010

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 55
-Paragraph 103
-URI http://asc.fasb.org/extlink&oid=65015998&loc=SL5199526-159011

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration