SEGMENTED INFORMATION (Details) - CAD CAD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segmented Information
Revenues	CAD 12,889	CAD 9,227	CAD 11,116	CAD 11,146	CAD 9,338	CAD 8,488	CAD 7,939	CAD 8,795	CAD 44,378	CAD 34,560	CAD 33,794
Commodity and gas distribution costs									(28,637)	(24,005)	(25,241)
Operating and administrative									(6,442)	(4,358)	(4,131)
Impairment of property, plant and equipment									(4,463)	(1,376)	(96)
Goodwill impairment									(102)	0	(440)
Income/(loss) from equity investments									1,102	428	475
Other income/(expense)									452	1,032	(702)
Earnings/(loss) before interest and income taxes									6,288	6,281	3,659
Depreciation and amortization									(3,163)	(2,240)	(2,024)
Interest expense									(2,556)	(1,590)	(1,624)
Income taxes									2,697	(142)	(170)
Earnings/(loss)									3,266	2,309	(159)
Capital expenditures									8,422	5,129	7,275
Total assets	162,093				85,209				162,093	85,209
Out of period adjustments
Out of period non-cash adjustment to income taxes											71
Liquids Pipelines
Segmented Information
Goodwill impairment									0	0
Gas Distribution
Segmented Information
Goodwill impairment									0	0
Green Power and Transmission
Segmented Information
Goodwill impairment									0	0
Operating Segments | Liquids Pipelines
Segmented Information
Revenues									8,913	8,176	5,589
Commodity and gas distribution costs									(18)	(12)	(9)
Operating and administrative									(2,949)	(2,908)	(2,748)
Impairment of property, plant and equipment									0	(1,365)	(80)
Goodwill impairment									0		0
Income/(loss) from equity investments									416	194	296
Other income/(expense)									33	841	(15)
Earnings/(loss) before interest and income taxes									6,395	4,926	3,033
Capital expenditures									2,799	3,957	5,884
Total assets	63,881				52,007				63,881	52,007
Operating Segments | Gas Transmission and Midstream
Segmented Information
Revenues									7,067	2,877	3,803
Commodity and gas distribution costs									(2,834)	(2,206)	(3,002)
Operating and administrative									(1,756)	(446)	(506)
Impairment of property, plant and equipment									(4,463)	(11)	(16)
Goodwill impairment									(102)		(440)
Income/(loss) from equity investments									653	223	200
Other income/(expense)									166	27	4
Earnings/(loss) before interest and income taxes									(1,269)	464	43
Capital expenditures									4,016	176	385
Total assets	60,745				11,182				60,745	11,182
Operating Segments | Gas Distribution
Segmented Information
Revenues									4,992	2,976	3,609
Commodity and gas distribution costs									(2,689)	(1,653)	(2,349)
Operating and administrative									(960)	(553)	(536)
Impairment of property, plant and equipment									0	0	0
Goodwill impairment									0		0
Income/(loss) from equity investments									23	12	(10)
Other income/(expense)									24	49	49
Earnings/(loss) before interest and income taxes									1,390	831	763
Capital expenditures									1,177	713	858
Total assets	25,956				10,132				25,956	10,132
Operating Segments | Green Power and Transmission
Segmented Information
Revenues									534	502	498
Commodity and gas distribution costs									0	5	4
Operating and administrative									(163)	(173)	(143)
Impairment of property, plant and equipment									0	0	0
Goodwill impairment									0		0
Income/(loss) from equity investments									6	2	2
Other income/(expense)									(5)	8	2
Earnings/(loss) before interest and income taxes									372	344	363
Capital expenditures									321	251	68
Total assets	6,289				5,571				6,289	5,571
Operating Segments | Energy Services
Segmented Information
Revenues									23,282	20,364	20,842
Commodity and gas distribution costs									(23,508)	(20,473)	(20,443)
Operating and administrative									(47)	(63)	(66)
Impairment of property, plant and equipment									0	0	0
Goodwill impairment									0		0
Income/(loss) from equity investments									8	(3)	(9)
Other income/(expense)									2	(8)	0
Earnings/(loss) before interest and income taxes									(263)	(183)	324
Capital expenditures									1	0	0
Total assets	2,514				1,951				2,514	1,951
Intersegment Eliminations | Eliminations and Other
Segmented Information
Revenues									(410)	(335)	(547)
Commodity and gas distribution costs									412	334	558
Operating and administrative									(567)	(215)	(132)
Impairment of property, plant and equipment									0	0	0
Goodwill impairment									0		0
Income/(loss) from equity investments									(4)	0	(4)
Other income/(expense)									232	115	(742)
Earnings/(loss) before interest and income taxes									(337)	(101)	(867)
Capital expenditures									108	32	CAD 80
Total assets	CAD 2,708				CAD 4,366				CAD 2,708	CAD 4,366
Customer Concentration Risk | Sales Revenue, Net | Largest Non-Affiliated Customer
Out of period adjustments
Concentration risk, percentage									11.80%	18.00%	21.80%
Customer Concentration Risk | Sales Revenue, Net | Second Largest Customer
Out of period adjustments
Concentration risk, percentage										10.40%
Customer Concentration Risk | Sales Revenue, Net | Third Largest Customer
Out of period adjustments
Concentration risk, percentage											10.80%