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DEBT - Components of Debt (Details) - USD ($)
$ in Thousands
Jul. 31, 2018
Jan. 31, 2018
Debt Instrument [Line Items]    
Convertible Debt, Noncurrent $ 429,598  
Convertible Debt    
Debt Instrument [Line Items]    
Convertible Debt Par Amount 575,000 $ 575,000
Debt Instrument, Unamortized Discount (137,719)  
Unamortized Debt Issuance Expense (7,683)  
Debt Instrument, Convertible, Carrying Amount of Equity Component, Excluding Equity Issuance Costs $ 148,598  
Debt Instrument, Interest Rate, Effective Percentage 6.62%  
Debt Instrument, Convertible, Transaction Costs Attributable to Equity Component $ 3,000