SHORT-TERM INVESTMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 548,034	$ 448,941
Gross Unrealized Gain	33	226
Gross Unrealized Loss	(738)	(235)
Fair Value	547,329	448,932
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity [Abstract]
Amortized cost, Due in 1 year or less	371,567
Amortized cost, Due in 1-2 years	176,467
Total amortized cost	548,034
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Fair value, Due in 1 year or less	371,489
Fair value, Due in 1-2 years	175,840
Total fair value	547,329
Bank time deposits
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	166,321	175,745
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	166,321	175,745
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	363,127	243,140
Gross Unrealized Gain	18	98
Gross Unrealized Loss	(729)	(219)
Fair Value	362,416	243,019
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	13,921	25,938
Gross Unrealized Gain	0	5
Gross Unrealized Loss	0	(7)
Fair Value	13,921	25,936
Mutual funds
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	4,665	4,118
Gross Unrealized Gain	15	123
Gross Unrealized Loss	(9)	(9)
Fair Value	$ 4,671	$ 4,232