REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Current regulatory assets	$ 232	$ 476
Noncurrent regulatory assets	5,397	4,636
Regulatory assets	5,629	5,112
Regulatory Liabilities [Line Items]
Current regulatory liabilities	270	243
Noncurrent regulatory liabilities	3,124	2,855
Regulatory liabilities	3,394	3,098
Purchased gas variance
Regulatory Liabilities [Line Items]
Current regulatory liabilities	153	41
Other Regulatory Assets (Liabilities)
Regulatory Liabilities [Line Items]
Current regulatory liabilities	117	202
Noncurrent regulatory liabilities	150	111
Future removal and restoration costs
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	1,455	1,424
U.S income Taxes
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	941	866
Pipeline future abandonment costs
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	578	454
Federal carbon receivables
Regulatory Assets [Line Items]
Current regulatory assets	0	145
Under-recovery of fuel costs
Regulatory Assets [Line Items]
Current regulatory assets	86	119
Deferred income taxes
Regulatory Assets [Line Items]
Noncurrent regulatory assets	3,890	3,551
Long-term debt
Regulatory Assets [Line Items]
Noncurrent regulatory assets	429	464
Pension plans and OPEB
Regulatory Assets [Line Items]
Noncurrent regulatory assets	402	275
Negative salvage
Regulatory Assets [Line Items]
Noncurrent regulatory assets	246	5
Accounting policy changes
Regulatory Assets [Line Items]
Noncurrent regulatory assets	169	175
Other Regulatory Assets (Liabilities)
Regulatory Assets [Line Items]
Current regulatory assets	146	212
Noncurrent regulatory assets	$ 261	$ 166