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ACQUISITIONS - Net Purchase Price Allocation for 2015 GTN Acquisition (Details 2) - USD ($)
$ in Millions
12 Months Ended
Apr. 01, 2015
Jul. 01, 2013
May. 22, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Net purchase price allocation            
Equity contribution       $ 2    
General Partner            
Net purchase price allocation            
Equity contribution $ 2     2    
GTN            
Net purchase price allocation            
Reduction in Partners' Equity       359    
GTN | General Partner            
Net purchase price allocation            
Reduction in Partners' Equity       $ 3    
GTN | Former parent, TransCanada subsidiaries | Transaction between entities under common control            
Net purchase price allocation            
Net Purchase Price 359          
Less: TransCanada's carrying value of non-controlling interest 232          
Excess purchase price 127 $ 262        
Purchase price 457 750        
Assumption of proportional debt 98 $ 146        
GTN | Former parent, TransCanada subsidiaries | Transaction between entities under common control | Common units and General Partner interest combined            
Net purchase price allocation            
Reduction in Partners' Equity $ 127          
TC PipeLines GP, Inc. | General Partner            
Net purchase price allocation            
Ownership interest in the Partnership (as a percent) 2.00%   2.00% 2.00% 2.00% 2.00%