XML 67 R50.htm IDEA: XBRL DOCUMENT v3.8.0.1
NET INCOME PER COMMON UNIT - General Partner Effective Interest and Allocated Incentive Distributions (Details)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
TC PipeLines GP, Inc. | General Partner    
PARTNERS' EQUITY    
General partner interest (as a percent) 2.00% 2.00%