RELATED PARTY TRANSACTIONS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
RELATED PARTY TRANSACTIONS
Amounts receivable from affiliates	$ 1,108	$ 1,023
Minimum
RELATED PARTY TRANSACTIONS
Annual interest rate on the loans (as a percent)	3.00%
Maximum
RELATED PARTY TRANSACTIONS
Annual interest rate on the loans (as a percent)	8.00%
Significantly Influenced Investees | Alliance Pipeline Limited | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	$ 0	0	$ 0
Significantly Influenced Investees | Alliance Pipeline Limited | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0	0	0
Significantly Influenced Investees | Alliance Pipeline Limited | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Alliance Pipeline Limited | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	81	112	93
Significantly Influenced Investees | Alliance Pipeline Limited | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Aux Sable Midstream LLC | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Aux Sable Midstream LLC | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0	0	0
Significantly Influenced Investees | Aux Sable Midstream LLC | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Aux Sable Midstream LLC | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	2	14	8
Significantly Influenced Investees | Aux Sable Midstream LLC | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Aux Sable Canada LP | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Aux Sable Canada LP | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0	0	0
Significantly Influenced Investees | Aux Sable Canada LP | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	61	72
Significantly Influenced Investees | Aux Sable Canada LP | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	91	272	189
Significantly Influenced Investees | Aux Sable Canada LP | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Seaway Crude Pipeline System | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Seaway Crude Pipeline System | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	342	327	309
Significantly Influenced Investees | Seaway Crude Pipeline System | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Seaway Crude Pipeline System | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	256	240	149
Significantly Influenced Investees | Seaway Crude Pipeline System | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Alliance Canada Marketing L.P. | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Alliance Canada Marketing L.P. | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0	0	0
Significantly Influenced Investees | Alliance Canada Marketing L.P. | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	64	106	125
Significantly Influenced Investees | Alliance Canada Marketing L.P. | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	17	46	49
Significantly Influenced Investees | Alliance Canada Marketing L.P. | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Nexus Gas Transmission, LLC | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	69	62	9
Significantly Influenced Investees | Nexus Gas Transmission, LLC | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	21	17	2
Significantly Influenced Investees | Nexus Gas Transmission, LLC | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Nexus Gas Transmission, LLC | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Nexus Gas Transmission, LLC | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	116	114	0
Significantly Influenced Investees | Vector Pipeline L.P. | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Vector Pipeline L.P. | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	7	7	7
Significantly Influenced Investees | Vector Pipeline L.P. | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Vector Pipeline L.P. | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	1
Significantly Influenced Investees | Vector Pipeline L.P. | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	19	19	20
Significantly Influenced Investees | Énergir, L.P. | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	37	38
Significantly Influenced Investees | Énergir, L.P. | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0	0
Significantly Influenced Investees | Énergir, L.P. | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0
Significantly Influenced Investees | Énergir, L.P. | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0
Significantly Influenced Investees | Énergir, L.P. | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0
Significantly Influenced Investees | DCP Midstream, LLC | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	3	4	5
Significantly Influenced Investees | DCP Midstream, LLC | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0	0	0
Significantly Influenced Investees | DCP Midstream, LLC | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	24	36	52
Significantly Influenced Investees | DCP Midstream, LLC | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | DCP Midstream, LLC | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Gulfstream Management and Operating Services, LLC | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Gulfstream Management and Operating Services, LLC | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	4	4	5
Significantly Influenced Investees | Gulfstream Management and Operating Services, LLC | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Gulfstream Management and Operating Services, LLC | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Gulfstream Management and Operating Services, LLC | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Sabal Trail Transmission, LLC | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Sabal Trail Transmission, LLC | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	25	23	18
Significantly Influenced Investees | Sabal Trail Transmission, LLC | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Sabal Trail Transmission, LLC | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Sabal Trail Transmission, LLC | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Steckman Ridge, LP | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Steckman Ridge, LP | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	4	4	4
Significantly Influenced Investees | Steckman Ridge, LP | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0	0	0
Significantly Influenced Investees | Steckman Ridge, LP | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	0	0
Significantly Influenced Investees | Steckman Ridge, LP | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0	$ 0	$ 0
Significantly Influenced Investees | Noverco | Transportation and other services
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	0
Significantly Influenced Investees | Noverco | Operating and Administrative
RELATED PARTY TRANSACTIONS
Operating and administrative	0
Significantly Influenced Investees | Noverco | Commodity Sales
RELATED PARTY TRANSACTIONS
Transportation and other services and Commodity sales	3
Significantly Influenced Investees | Noverco | Commodity Costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	0
Significantly Influenced Investees | Noverco | Gas Distribution costs
RELATED PARTY TRANSACTIONS
Commodity costs and Gas distribution costs	$ 0