STOCK OPTION AND STOCK UNIT PLANS - STOCK OPTION ACTIVITY (Details) - CAD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
STOCK OPTION AND STOCK UNIT PLANS
Compensation expense	$ 145	$ 117	$ 106
INCENTIVE STOCK OPTIONS
STOCK OPTION AND STOCK UNIT PLANS
Compensation expense	$ 24	$ 32	28
Vesting period	4 years
Expiration term	10 years
Number
Options outstanding at beginning of year (in shares)	35,047
Options granted (in shares)	4,783
Options exercised (in shares)	(2,656)
Options cancelled or expired (in shares)	(1,680)
Options outstanding at end of year (in shares)	35,494	35,047
Options vested at end of year (in shares)	22,005
Weighted Average Exercise Price
Options outstanding at beginning of year (in dollars per share)	$ 47.73
Options granted (in dollars per share)	55.50
Options exercised (in dollars per share)	37.12
Options cancelled or expired (in dollars per share)	52.43
Options outstanding at end of year (in dollars per share)	49.35	$ 47.73
Options vested at end of year (in dollars per share)	$ 48.65
Weighted Average Remaining Contractual Life (years)
Options outstanding at end of year	6 years
Options vested at end of year	4 years 7 months 6 days
Aggregate Intrinsic Value
Options outstanding at end of year	$ 54
Options vested at end of year	34
Stock options, additional disclosures
Total intrinsic value of awards exercised	13	$ 58	42
Cash received on exercise of awards	4	1	15
Total fair value of options vested	$ 30	$ 32	$ 36