FAIR VALUE MEASUREMENTS - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 28,325	$ 31,703
Accumulated gross unrealized gain, before tax	12	10
Accumulated gross unrealized loss, before tax	(306)	(464)
Debt securities, available-for-sale	86,507	120,559
Cash, cash equivalents and debt securities available-for-sale, amortized cost	115,126	152,716
Cash, cash equivalents and debt securities available-for-sale	114,832	152,262
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	28,325	31,703
Cash and cash equivalents, fair value	28,325	31,703
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	86,801	121,013
Accumulated gross unrealized gain, before tax	12	10
Accumulated gross unrealized loss, before tax	(306)	(464)
Debt securities, available-for-sale	86,507	120,559
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost		13,254
Accumulated gross unrealized gain, before tax		0
Accumulated gross unrealized loss, before tax		(8)
Debt securities, available-for-sale		13,246
Corporate debt | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	59,480	70,631
Accumulated gross unrealized gain, before tax	6	6
Accumulated gross unrealized loss, before tax	(222)	(296)
Debt securities, available-for-sale	59,264	70,341
Municipal securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	5,504	3,385
Accumulated gross unrealized gain, before tax	1	3
Accumulated gross unrealized loss, before tax	(4)	(1)
Debt securities, available-for-sale	5,501	3,387
Asset backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	17,577	27,063
Accumulated gross unrealized gain, before tax	5	1
Accumulated gross unrealized loss, before tax	(50)	(119)
Debt securities, available-for-sale	17,532	26,945
Agency bond | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	4,240	4,183
Accumulated gross unrealized gain, before tax	0	0
Accumulated gross unrealized loss, before tax	(30)	(35)
Debt securities, available-for-sale	4,210	4,148
International government securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost		2,497
Accumulated gross unrealized gain, before tax		0
Accumulated gross unrealized loss, before tax		(5)
Debt securities, available-for-sale		2,492
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	22,005	16,499
Cash and cash equivalents, fair value	22,005	16,499
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	6,320	15,204
Cash and cash equivalents, fair value	$ 6,320	$ 15,204