LONG-TERM INVESTMENTS - Schedule of Long-Term Investments (Details) CAD in Millions, $ in Billions	Dec. 31, 2017 CAD	Apr. 27, 2017	Feb. 15, 2017 CAD	Feb. 15, 2017 USD ($)	Feb. 08, 2017	Dec. 31, 2016 CAD	May 19, 2016
LONG-TERM INVESTMENTS
Total long-term investments	CAD 16,644					CAD 6,836
Noverco | Common shares
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	38.90%					38.90%
Noverco | Common shares | Gas Distribution
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	38.90%
EQUITY INVESTMENTS	CAD 0					CAD 0
Noverco | Preference shares | Gas Distribution
LONG-TERM INVESTMENTS
OTHER LONG-TERM INVESTMENTS	CAD 371					355
Hohe See Offshore Wind Project
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)					50.00%
Bakken Pipeline System
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)		27.60%
Bakken Pipeline System | EEP
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)			27.60%	27.60%
Purchase price			CAD 2,000	$ 1.5
Bakken Pipeline System | Liquids Pipelines
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	27.60%
EQUITY INVESTMENTS	CAD 1,938					CAD 0
Eddystone Rail Company, LLC | Liquids Pipelines
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	100.00%					75.00%
EQUITY INVESTMENTS	CAD 0					CAD 19
Seaway | Liquids Pipelines
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 2,882					3,129
Illinois Extension Pipeline Company, L.L.C.2 | Liquids Pipelines
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	65.00%
EQUITY INVESTMENTS	CAD 686					759
Alliance Pipeline | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 375					411
Aux Sable | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
EQUITY INVESTMENTS	CAD 300					324
Aux Sable | Gas Transmission and Midstream | Minimum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	42.70%
Aux Sable | Gas Transmission and Midstream | Maximum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
DCP Midstream, LLC | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 2,143					0
Gulfstream Natural Gas System, L.L.C. | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 1,205					0
Nexus Gas Transmission, LLC | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 834					0
Offshore - various joint ventures | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
EQUITY INVESTMENTS	CAD 389					435
Offshore - various joint ventures | Gas Transmission and Midstream | Minimum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	22.00%
Offshore - various joint ventures | Gas Transmission and Midstream | Maximum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	74.30%
PennEast Pipeline Company, LLC | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	20.00%
EQUITY INVESTMENTS	CAD 69					0
Sabal Trail Transmission, LLC | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 2,355					0
Southeast Supply Header L.L.C. | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 486					0
Steckman Ridge LP | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	49.50%
EQUITY INVESTMENTS	CAD 221					0
Texas Express Pipeline | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	35.00%
EQUITY INVESTMENTS	CAD 430					484
Vector Pipeline L.P. | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	60.00%
EQUITY INVESTMENTS	CAD 169					159
Eolien Maritime France SAS
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)							50.00%
Eolien Maritime France SAS | Green Power and Transmission
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 69					58
Hohe See Offshore Wind Project | Green Power and Transmission
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 763					0
Rampion Offshore Wind Project | Green Power and Transmission
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	24.90%
EQUITY INVESTMENTS	CAD 555					345
Emerging Technologies and Other | Green Power and Transmission
LONG-TERM INVESTMENTS
OTHER LONG-TERM INVESTMENTS	80					90
Other equity investments | Liquids Pipelines
LONG-TERM INVESTMENTS
EQUITY INVESTMENTS	CAD 87					70
Other equity investments | Liquids Pipelines | Minimum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	30.00%
Other equity investments | Liquids Pipelines | Maximum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	43.80%
Other equity investments | Gas Transmission and Midstream
LONG-TERM INVESTMENTS
EQUITY INVESTMENTS	CAD 34					4
Other equity investments | Gas Transmission and Midstream | Minimum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	33.30%
Other equity investments | Gas Transmission and Midstream | Maximum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	70.00%
Other equity investments | Green Power and Transmission
LONG-TERM INVESTMENTS
EQUITY INVESTMENTS	CAD 95					100
Other equity investments | Green Power and Transmission | Minimum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	19.00%
Other equity investments | Green Power and Transmission | Maximum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
Other equity investments | Eliminations and Other
LONG-TERM INVESTMENTS
EQUITY INVESTMENTS	CAD 26					15
Other equity investments | Eliminations and Other | Minimum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	19.00%
Other equity investments | Eliminations and Other | Maximum
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	42.70%
Other equity investments | Common shares | Gas Distribution
LONG-TERM INVESTMENTS
Ownership Interest (as a percent)	50.00%
EQUITY INVESTMENTS	CAD 15					0
Other long-term investments | Eliminations and Other
LONG-TERM INVESTMENTS
OTHER LONG-TERM INVESTMENTS	CAD 67					CAD 79