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SHORT-TERM INVESTMENTS (Contracted Maturities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Mar. 31, 2018
Debt Securities, Available-for-sale, Maturity, Amortized Cost, Rolling Maturity [Abstract]    
Amortized cost, Due in 1 year or less $ 429,993  
Amortized cost, Due in 1-2 years 116,531  
Cost or Amortized Cost 546,524 $ 617,318
Debt Securities, Available-for-sale, Maturity, Fair Value, Rolling Maturity [Abstract]    
Fair value, Due in 1 year or less 429,296  
Fair value, Due in 1-2 years 116,438  
Total fair value $ 545,734 $ 615,406