REVENUES - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 4,294	$ 4,309	$ 9,685	$ 9,845
Total revenues	10,948	7,956	23,085	19,969
Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	17	15	34	30
Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,277	4,294	9,651	9,815
Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,353	3,418	7,019	7,328
Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	151	146	309	298
Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10	5	17	12
Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	725	686	2,259	2,103
Total revenues	737	694	2,277	2,110
Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55	54	81	104
Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	6,334	2,936	12,763	10,325
Total revenues	6,334	2,936	12,763	10,325
Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	320	711	637	(201)
Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,194	2,165	4,549	4,631
Liquids Pipelines | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Liquids Pipelines | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,194	2,165	4,549	4,631
Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,119	1,197	2,321	2,538
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	0	0
Gas Transmission and Midstream | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,119	1,197	2,321	2,538
Gas Distribution and Storage
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	926	893	2,734	2,572
Gas Distribution and Storage | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	17	15	34	30
Gas Distribution and Storage | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	909	878	2,700	2,542
Renewable Power Generation
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55	54	81	104
Renewable Power Generation | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Renewable Power Generation | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55	54	81	104
Business segments | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,194	2,165	4,549	4,631
Total revenues	2,547	2,945	5,246	4,479
Business segments | Liquids Pipelines | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,157	2,141	4,486	4,581
Business segments | Liquids Pipelines | Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	37	24	63	50
Business segments | Liquids Pipelines | Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Liquids Pipelines | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Liquids Pipelines | Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Liquids Pipelines | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Business segments | Liquids Pipelines | Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	215	598	427	(419)
Business segments | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,119	1,197	2,321	2,538
Total revenues	1,128	1,203	2,342	2,560
Business segments | Gas Transmission and Midstream | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,046	1,126	2,167	2,381
Business segments | Gas Transmission and Midstream | Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	63	66	137	145
Business segments | Gas Transmission and Midstream | Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10	5	17	12
Business segments | Gas Transmission and Midstream | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Gas Transmission and Midstream | Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Gas Transmission and Midstream | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Business segments | Gas Transmission and Midstream | Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	9	5	21	21
Business segments | Gas Distribution and Storage
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	926	893	2,734	2,572
Total revenues	953	903	2,776	2,585
Business segments | Gas Distribution and Storage | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	150	151	366	366
Business segments | Gas Distribution and Storage | Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	51	56	109	103
Business segments | Gas Distribution and Storage | Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Gas Distribution and Storage | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	725	686	2,259	2,103
Business segments | Gas Distribution and Storage | Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Gas Distribution and Storage | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Business segments | Gas Distribution and Storage | Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	12	8	18	7
Business segments | Renewable Power Generation
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55	54	81	104
Total revenues	132	150	249	303
Business segments | Renewable Power Generation | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Renewable Power Generation | Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Renewable Power Generation | Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Renewable Power Generation | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Renewable Power Generation | Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55	54	81	104
Business segments | Renewable Power Generation | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Business segments | Renewable Power Generation | Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	77	96	168	199
Business segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Total revenues	6,345	2,949	12,778	10,347
Business segments | Energy Services | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	6,334	2,936	12,763	10,325
Business segments | Energy Services | Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	1	11	1	4
Eliminations and Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Total revenues	(157)	(194)	(306)	(305)
Eliminations and Other | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Storage and other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Gas gathering and processing revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Electricity and transmission revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Eliminations and Other | Other revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	6	(7)	2	(13)
Intersegment revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(163)	(187)	(308)	(292)
Total revenues	(157)	(194)	(306)	(305)
Intersegment revenue | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	138	182	270	267
Intersegment revenue | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	1	0	1
Intersegment revenue | Gas Distribution and Storage
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	15	2	24	6
Intersegment revenue | Renewable Power Generation
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Intersegment revenue | Energy Services
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	$ 10	$ 2	$ 14	$ 18