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DEBT - Convertible Senior Notes Narrative (Details)
12 Months Ended
Aug. 11, 2016
USD ($)
trading_day
$ / shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]        
Interest expense   $ 6,964,000 $ 4,425,000 $ 941,000
Convertible Notes Payable [Member]        
Debt Instrument [Line Items]        
Debt issued $ 50,000,000      
Debt Instrument, Interest Rate, Stated Percentage 4.125%      
Proceeds from Convertible Debt $ 47,091,000      
Debt issuance cost $ 2,909,000      
Threshold Trading Days | trading_day 20      
Threshold Consecutive Trading Days | trading_day 30      
Threshold Percentage of Stock Price Trigger 130.00%      
Measurement Period 5 days      
Maximum Percentage of Trading Price Trigger 98.00%      
Conversion Ratio (in shares per $1,000 principal amount of Notes) 0.0457603      
Conversion Price (in dollars per share) | $ / shares $ 21.85      
Long-term Debt, Fair Value $ 38,334,000 45,294,000 39,949,000  
Debt Instrument, Convertible, Beneficial Conversion Feature 11,666,000      
Interest expense   4,344,000 1,604,000 0
Interest accrued   2,063,000 796,000 0
Accretion of preferred stock to fair value   2,281,000 $ 807,000 $ 0
Long-term Debt [Member] | Accounting Standards Update 2015-03 [Member] | Convertible Notes Payable [Member]        
Debt Instrument [Line Items]        
Debt issuance cost $ (2,228,000)      
Additional Paid-in Capital [Member]        
Debt Instrument [Line Items]        
Debt issuance costs allocated to equity   $ 680,000