SHAREHOLDERS' EQUITY - Share-Based Compensation (Details) - CAD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation
Share-based compensation expense	$ 23,321	$ 25,840	$ 23,757
Cash Settled DSU
Share-based Compensation
Share-based compensation expense	689	133	997
Equity-settled Long-term Incentive Plans
Share-based Compensation
Share-based compensation expense	22,324	25,917	22,576
Equity Swaps
Share-based Compensation
Share-based compensation expense	$ 308	$ (210)	$ 184

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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