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SHARE-BASED COMPENSATION - Summary of PSU activity (Details) - Performance share units - shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of units, beginning of period (in shares) 15,480 9,618
Grants (in shares) 0 6,058
Redeemed (in shares) (1,876) 0
Forfeited (in shares) 0 (196)
Number of units, end of period (in shares) 13,604 15,480