STOCK OPTION AND STOCK UNIT PLANS - STOCK OPTION ACTIVITY (Details) - INCENTIVE STOCK OPTIONS - CAD CAD / shares in Units, shares in Thousands, CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
STOCK OPTION AND STOCK UNIT PLANS
Vesting period	4 years
Expiration term	10 years
Number
Options outstanding at beginning of year (in shares)	32,909
Options granted (in shares)	5,995
Options exercised (in shares)	(3,350)
Options cancelled or expired (in shares)	(1,188)
Options outstanding at end of year (in shares)	34,366	32,909
Options vested at end of year (in shares)	20,403
Weighted Average Exercise Price
Options outstanding at beginning of year (in dollars per share)	CAD 42.51
Options granted (in dollars per share)	55.72
Options exercised (in dollars per share)	32.65
Options cancelled or expired (in dollars per share)	53.23
Options outstanding at end of year (in dollars per share)	45.41	CAD 42.51
Options vested at end of year (in dollars per share)	CAD 40.89
Weighted Average Remaining Contractual Life (years)
Options outstanding at end of year	6 years 1 month 6 days
Options vested at end of year	4 years 8 months 12 days
Aggregate Intrinsic Value
Options outstanding at end of year	CAD 271
Options vested at end of year	228
Stock options, additional disclosures
Total intrinsic value of awards exercised	62	CAD 123	CAD 126
Cash received on exercise of awards	17	37	43
Total fair value of options vested	CAD 44	CAD 36	CAD 34