LONG-TERM INVESTMENTS - Summary of Combined Financial Information (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income statement information
Operating revenues	CAD 16,213	CAD 4,102	CAD 3,887
Operating expenses	13,197	3,344	2,473
Earnings	2,728	641	1,078
Earnings attributable to controlling interests	1,262	469	490
Balance sheet information
Current assets	3,538	928
Non-current assets	45,026	15,915
Current liabilities	3,454	1,003
Non-current liabilities	13,595	5,134
Noncontrolling interests	3,191	0
Seaway
Income statement information
Operating revenues	959	938	833
Operating expenses	286	293	263
Earnings	672	643	566
Earnings attributable to controlling interests	336	322	283
Balance sheet information
Current assets	106	86
Non-current assets	3,329	3,651
Current liabilities	143	172
Non-current liabilities	13	13
Noncontrolling interests	0	0
Other
Income statement information
Operating revenues	15,254	3,164	3,054
Operating expenses	12,911	3,051	2,210
Earnings	2,056	(2)	512
Earnings attributable to controlling interests	926	147	CAD 207
Balance sheet information
Current assets	3,432	842
Non-current assets	41,697	12,264
Current liabilities	3,311	831
Non-current liabilities	13,582	5,121
Noncontrolling interests	CAD 3,191	CAD 0