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LONG-TERM DEBT - Contractual Coupon Interest Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Disclosure [Abstract]      
Contractual coupon interest expense $ 470 $ 1,570 $ 3,198
Accretion of discount and fees on the liability component $ 461 $ 1,537 $ 2,902