REVENUES - Disaggregation of Revenue (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 4,275	$ 4,424	$ 14,583	$ 15,832
Total revenues	11,598	11,345	37,717	34,816
Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	17	318	54	1,695
Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,258	4,106	14,529	14,137
Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,513	3,266	10,627	9,703
Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148	139	405	450
Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	98	200	329	636
Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	470	478	3,080	3,260
Total revenues	454	478	3,085	3,260
Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	46	43	139	153
Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		298	3	1,630
Revenue not from contract with customers	7,396	6,621	22,441	19,008
Total revenues	7,396	6,919	22,444	20,638
Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(73)	300	693	(24)
Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,336	2,221	6,832	6,440
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	2,336	2,221	6,832	6,440
Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,240	1,530	3,823	5,319
Gas Transmission and Midstream | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	298	3	1,630
Gas Transmission and Midstream | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,240	1,232	3,820	3,689
Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	653	630	3,789	3,920
Gas Distribution | Revenues from products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	17	20	51	65
Gas Distribution | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	636	610	3,738	3,855
Renewable Power Generation and Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	46	43	139	153
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	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	46	43	139	153
Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	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	0	0
Energy Services | Revenues from products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,336	2,221	6,832	6,440
Total revenues	2,268	2,529	7,495	6,388
Business segments | Liquids Pipelines | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,305	2,190	6,749	6,327
Business segments | Liquids Pipelines | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	31	31	83	113
Business segments | Liquids Pipelines | Gas gathering and processing revenues
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 revenues
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 from contract with customer		0	0	0
Revenue not from contract with customers	0	0	0	0
Business segments | Liquids Pipelines | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(156)	222	383	(308)
Business segments | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,240	1,530	3,823	5,319
Total revenues	1,264	1,528	3,870	5,329
Business segments | Gas Transmission and Midstream | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,073	979	3,323	2,889
Business segments | Gas Transmission and Midstream | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	69	53	168	164
Business segments | Gas Transmission and Midstream | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	98	200	329	636
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 revenues
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 from contract with customer		298	3	1,630
Revenue not from contract with customers	0	0	0	0
Business segments | Gas Transmission and Midstream | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	23	(6)	43	2
Business segments | Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	653	630	3,789	3,920
Total revenues	635	645	3,803	3,952
Business segments | Gas Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	135	97	555	487
Business segments | Gas Distribution | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	48	55	154	173
Business segments | Gas Distribution | Gas gathering and processing revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Gas Distribution | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	470	478	3,080	3,260
Business segments | Gas Distribution | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Gas Distribution | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		0	0	0
Revenue not from contract with customers	0	0	0	0
Business segments | Gas Distribution | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(21)	11	5	22
Business segments | Renewable Power Generation and Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	46	43	139	153
Total revenues	128	117	417	423
Business segments | Renewable Power Generation and Transmission | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	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	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	0	0
Business segments | Renewable Power Generation and Transmission | Gas distribution revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Renewable Power Generation and Transmission | Electricity and transmission revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	46	43	139	153
Business segments | Renewable Power Generation and Transmission | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer		0	0	0
Revenue not from contract with customers	0	0	0	0
Business segments | Renewable Power Generation and Transmission | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	82	74	278	270
Business segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Total revenues	7,403	6,646	22,494	19,114
Business segments | Energy Services | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Business segments | Energy Services | Gas gathering and processing revenues
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 revenues
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 from contract with customer		0	0	0
Revenue not from contract with customers	7,396	6,621	22,441	19,008
Business segments | Energy Services | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(1)	0	(2)	0
Eliminations and Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Total revenues	(100)	(120)	(362)	(390)
Eliminations and Other | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Storage and other revenues
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0	0	0	0
Eliminations and Other | Gas gathering and processing revenues
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 revenues
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 from contract with customer		0	0	0
Revenue not from contract with customers	0	0	0	0
Eliminations and Other | Other revenues
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	(1)	(14)	(10)
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(100)	(119)	(348)	(380)
Total revenues	(100)	(120)	(362)	(390)
Intersegment Eliminations | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	88	86	280	256
Intersegment Eliminations | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	1	4	4	8
Intersegment Eliminations | Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	3	4	9	10
Intersegment Eliminations | Renewable Power Generation and Transmission
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0	0	0	0
Intersegment Eliminations | Energy Services
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	$ 8	$ 25	$ 55	$ 106