INCOME TAXES - UNITED STATES TAX REFROM (Details) - CAD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States BEAT tax			$ 67	$ 43	$ 0
Impact of United States tax reform	$ 223	$ 30	$ 0	$ 2	2,045
Provisional tax expense					34
Decrease in deferred income tax liability					3,100
Deferred income tax asset benefit					1,100
Reduction in tax amount					$ 200