LONG-TERM INVESTMENTS - Summary of Combined Financial Information (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income statement information
Revenues	$ 10,008	$ 9,110	$ 7,956	$ 12,013	$ 12,352	$ 11,598	$ 13,263	$ 12,856	$ 39,087	$ 50,069	$ 46,378
Earnings									3,416	5,827	3,333
Earnings attributable to controlling interests	1,871	$ 1,084	$ 1,741	$ (1,333)	842	$ 1,045	$ 1,832	$ 1,986	3,363	5,705	2,882
Balance sheet information
Current assets	7,350				8,713				7,350	8,713
Current liabilities	13,979				15,898				13,979	15,898
Noncontrolling interests	2,996				3,364				2,996	3,364
Unconsolidated equity method investments
Income statement information
Revenues									13,987	15,687	19,217
Cost of goods and services sold									12,223	13,153	15,634
Earnings									2,306	3,016	2,954
Earnings attributable to controlling interests									1,136	1,503	$ 1,509
Balance sheet information
Current assets	3,136				2,481				3,136	2,481
Non-current assets	45,955				48,942				45,955	48,942
Current liabilities	3,539				4,047				3,539	4,047
Non-current liabilities	19,639				18,126				19,639	18,126
Noncontrolling interests	$ 3,810				$ 2,779				$ 3,810	$ 2,779