SIGNIFICANT ACCOUNTING POLICIES - Ownership Interests Acquired (Details)	Dec. 31, 2016	Jan. 01, 2016	Apr. 01, 2015	Mar. 31, 2015	Oct. 01, 2014	Sep. 30, 2014
Former parent, TransCanada subsidiaries | Portland Natural Gas Transmission System | Transaction between entities under common control
Acquisitions
Interest acquired (as a percent)		49.90%
TransCanada | Portland Natural Gas Transmission System
Acquisitions
Remaining noncontrolling ownership interest (as a percent)	11.81%
TransCanada | GTN
Acquisitions
Remaining noncontrolling ownership interest (as a percent)				30.00%
TransCanada | Bison
Acquisitions
Remaining noncontrolling ownership interest (as a percent)						30.00%
GTN | Former parent, TransCanada subsidiaries | Transaction between entities under common control
Acquisitions
Interest acquired (as a percent)			30.00%
Bison | Former parent, TransCanada subsidiaries | Transaction between entities under common control
Acquisitions
Interest acquired (as a percent)	30.00%		30.00%		30.00%