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ACQUISITIONS - Acquisition of Ownership Interest in PNGTS (Details) - Portland Natural Gas Transmission System - USD ($)
$ in Millions
Jan. 01, 2016
Dec. 31, 2016
Acquisition    
Interest acquired (as a percent) 49.90% 49.90%
TransCanada | Transaction between entities under common control    
Acquisition    
Total purchase price $ 228  
Net purchase price 193  
Less: TransCanada's carrying value of non-controlling interest 120  
Excess purchase price 73  
Purchase price adjustments 5  
Assumption of proportional debt 35  
Additional contingent payment, minimum 5  
Additional contingent payment, maximum $ 50  
Period following closing date during which additional payments may be required 15 years