STOCK OPTION AND STOCK UNIT PLANS - STOCK OPTION ACTIVITY (Details) - INCENTIVE STOCK OPTIONS - CAD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
STOCK OPTION AND STOCK UNIT PLANS
Vesting period	4 years
Expiration term	10 years
Number
Options outstanding at beginning of year (in shares)	34,387
Options granted (in shares)	6,777
Options exercised (in shares)	(4,519)
Options cancelled or expired (in shares)	(1,598)
Options outstanding at end of year (in shares)	35,047	34,387
Options vested at end of year (in shares)	20,581
Weighted Average Exercise Price
Options outstanding at beginning of year (in dollars per share)	$ 43.47
Options granted (in dollars per share)	48.32
Options exercised (in dollars per share)	34.19
Options cancelled or expired (in dollars per share)	50.62
Options outstanding at end of year (in dollars per share)	47.73	$ 43.47
Options vested at end of year (in dollars per share)	$ 47.67
Weighted Average Remaining Contractual Life (years)
Options outstanding at end of year	6 years 2 months 12 days
Options vested at end of year	4 years 8 months 12 days
Aggregate Intrinsic Value
Options outstanding at end of year	$ 157
Options vested at end of year	92
Stock options, additional disclosures
Total intrinsic value of awards exercised	58	$ 42	$ 62
Cash received on exercise of awards	1	15	17
Total fair value of options vested	$ 32	$ 36	$ 44