REVENUE (Details) $ in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018 CAD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 CAD ($)	Jun. 30, 2018 CAD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 CAD ($)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 5,207			$ 11,600
Revenues	10,745		$ 11,116	23,471		$ 22,262
Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	659			1,377
Transferred over Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,548			10,223
Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,188			6,437
Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	145			311
Gas Gathering and Processing Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	231			436
Gas Distribution Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	856			2,782
Revenues	856		847	2,782		2,210
Electricity and Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148			302
Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	639			1,332
Revenue not from contract with customers	5,812			12,387
Revenues	6,451		6,620	13,719		13,486
Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(274)			(516)
Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,121			4,219
Liquids Pipelines | Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Liquids Pipelines | Transferred over Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,121			4,219
Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,879			3,789
Gas Transmission and Midstream | Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	639			1,332
Gas Transmission and Midstream | Transferred over Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,240			2,457
Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,059			3,290
Gas Distribution | Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20			45
Gas Distribution | Transferred over Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,039			3,245
Green Power and Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148			302
Green Power and Transmission | Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Green Power and Transmission | Transferred over Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148			302
Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Energy Services | Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Energy 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	2,121			4,219
Revenues	1,950		2,243	3,859		4,398
Business segments | Liquids Pipelines | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,079			4,137
Business segments | Liquids Pipelines | Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	42			82
Business segments | Liquids Pipelines | Gas Gathering and Processing Revenue
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 Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Liquids Pipelines | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenue not from contract with customers	0			0
Business segments | Liquids Pipelines | Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(261)			(530)
Business segments | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,879			3,789
Revenues	1,864		1,954	3,801		3,189
Business segments | Gas Transmission and Midstream | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	958			1,910
Business segments | Gas Transmission and Midstream | Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	51			111
Business segments | Gas Transmission and Midstream | Gas Gathering and Processing Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	231			436
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 Revenue
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 from contract with customer	639			1,332
Revenue not from contract with customers	0			0
Business segments | Gas Transmission and Midstream | Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(17)			8
Business segments | Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,059			3,290
Revenues	1,070			3,307
Business segments | Gas Distribution | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	151			390
Business segments | Gas Distribution | Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	52			118
Business segments | Gas Distribution | Gas Gathering and Processing Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Gas Distribution | Gas Distribution Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	856			2,782
Business segments | Gas Distribution | Electricity and Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Gas Distribution | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenue not from contract with customers	0			0
Business segments | Gas Distribution | Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	9			11
Business segments | Green Power and Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148			302
Revenues	149		140	306		277
Business segments | Green Power and Transmission | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Green Power and Transmission | Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Green Power and Transmission | Gas Gathering and Processing Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Green Power and Transmission | Gas Distribution Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Green Power and Transmission | Electricity and Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148			302
Business segments | Green Power and Transmission | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenue not from contract with customers	0			0
Business segments | Green Power and Transmission | Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	1			4
Business segments | Energy Services
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenues	5,836			12,468
Business segments | Energy Services | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Energy Services | Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Energy Services | Gas Gathering and Processing Revenue
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 Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Business segments | Energy Services | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenue not from contract with customers	5,812			12,387
Business segments | Energy Services | Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0			0
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	(118)	$ 0		(261)	$ 0
Intersegment Eliminations | Liquids Pipelines
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	90			170
Intersegment Eliminations | Gas Transmission and Midstream
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	2			4
Intersegment Eliminations | Gas Distribution
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	2			6
Intersegment Eliminations | Green Power and Transmission
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	0			0
Intersegment Eliminations | Energy Services
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	24			81
Consolidation, Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenues	(124)		$ (98)	(270)		$ (196)
Consolidation, Eliminations | Transportation revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Consolidation, Eliminations | Storage and Other Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Consolidation, Eliminations | Gas Gathering and Processing Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Consolidation, Eliminations | Gas Distribution Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Consolidation, Eliminations | Electricity and Transmission Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Consolidation, Eliminations | Commodity sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0			0
Revenue not from contract with customers	0			0
Consolidation, Eliminations | Other Revenue
Disaggregation of Revenue [Line Items]
Revenue not from contract with customers	$ (6)			$ (9)