REGULATORY MATTERS - Enbridge Gas Distribution Inc. (Narrative) (Details) - Enbridge Gas Distribution	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
After-tax rate of return on common equity embedded in rates (as a percent)	8.80%	9.20%
Common equity component of capital (as a percent)	36.00%	36.00%

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