REVENUES - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 5,536	$ 5,756
Total revenues	12,013	12,856
Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	15	17
Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,521	5,739
Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,910	3,600
Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	152	134
Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7	116
Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,417	1,856
Total revenues	1,416	1,876
Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50	50
Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	7,389	6,632
Total revenues	7,389	6,632
Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(912)	468
Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,466	2,241
Liquids Pipelines | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Liquids Pipelines | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,466	2,241
Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,341	1,306
Gas Transmission and Midstream | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Gas Transmission and Midstream | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,341	1,306
Gas Distribution and Storage
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,679	2,159
Gas Distribution and Storage | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	15	17
Gas Distribution and Storage | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,664	2,142
Renewable Power Generation and Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50	50
Renewable Power Generation and Transmission | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Renewable Power Generation and Transmission | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50	50
Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Energy Services | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Energy Services | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,466	2,241
Total revenues	1,534	2,658
Business segments | Liquids Pipelines | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,440	2,214
Business segments | Liquids Pipelines | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	26	27
Business segments | Liquids Pipelines | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Liquids Pipelines | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Liquids Pipelines | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Liquids Pipelines | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0
Business segments | Liquids Pipelines | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(1,017)	340
Business segments | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,341	1,306
Total revenues	1,357	1,318
Business segments | Gas Transmission and Midstream | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,255	1,137
Business segments | Gas Transmission and Midstream | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	79	53
Business segments | Gas Transmission and Midstream | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7	116
Business segments | Gas Transmission and Midstream | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Gas Transmission and Midstream | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Gas Transmission and Midstream | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0
Business segments | Gas Transmission and Midstream | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	16	10
Business segments | Gas Distribution and Storage
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,679	2,159
Total revenues	1,682	2,191
Business segments | Gas Distribution and Storage | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	215	249
Business segments | Gas Distribution and Storage | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	47	54
Business segments | Gas Distribution and Storage | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Gas Distribution and Storage | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,417	1,856
Business segments | Gas Distribution and Storage | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Gas Distribution and Storage | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0
Business segments | Gas Distribution and Storage | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(1)	29
Business segments | Renewable Power Generation and Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50	50
Total revenues	153	152
Business segments | Renewable Power Generation and Transmission | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Renewable Power Generation and Transmission | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Renewable Power Generation and Transmission | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Renewable Power Generation and Transmission | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Renewable Power Generation and Transmission | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	50	50
Business segments | Renewable Power Generation and Transmission | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0
Business segments | Renewable Power Generation and Transmission | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	103	102
Business segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Total revenues	7,398	6,673
Business segments | Energy Services | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Energy Services | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Energy Services | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Energy Services | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Energy Services | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Business segments | Energy Services | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	7,389	6,632
Business segments | Energy Services | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(7)	6
Eliminations and Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Total revenues	(111)	(136)
Eliminations and Other | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Eliminations and Other | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Eliminations and Other | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Eliminations and Other | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Eliminations and Other | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0
Eliminations and Other | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0
Eliminations and Other | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(6)	(19)
Intersegment revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(105)	(117)
Total revenues	(111)	(136)
Intersegment revenues | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	85	77
Intersegment revenues | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	2
Intersegment revenues | Gas Distribution and Storage
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	4	3
Intersegment revenues | Renewable Power Generation and Transmission
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0
Intersegment revenues | Energy Services
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	$ 16	$ 35