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STOCK-BASED COMPENSATION - Unvested Restricted Stock Units Outstanding (Details) - Time Lapse Restricted Shares - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Unvested 2,261 2,751 3,100
Forfeited (in shares) (34) (114) (85)
Vested (in shares) (679) (879) (946)
Granted (in shares) 469 503 682
Unvested 2,017 2,261 2,751
Weighted-Average Grant-Date Fair Value      
Unvested (in dollars per share) $ 20.21 $ 17.21 $ 14.45
Forfeited (in dollars per share) 22.38 19.54 15.71
Vested (in dollars per share) 17.21 14.49 12.04
Granted (in dollars per share) 34.46 26.45 22.43
Unvested (in dollars per share) $ 24.50 $ 20.21 $ 17.21