PENSION AND OTHER POSTRETIREMENT BENEFITS - RETIREMENT SAVINGS PLAN (Details) - CAD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Canada
Pension and Other Postretirement Benefit Disclosures
Percent of match	5.00%
Total contributions by the Company	$ 13,000,000	$ 14,000,000	$ 0
United States
Pension and Other Postretirement Benefit Disclosures
Percent of match	6.00%
Total contributions by the Company	$ 27,000,000	$ 31,000,000	$ 13,000,000

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration