FAIR VALUE MEASUREMENTS - Cash, Cash Equivalents and Investments with Hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 276,583	$ 31,703
Accumulated gross unrealized gain, before tax	77	10
Accumulated gross unrealized loss, before tax	(61)	(464)
Debt securities, available-for-sale	69,899	120,559
Cash, cash equivalents and debt securities available-for-sale, amortized cost	346,466	152,716
Cash, cash equivalents and debt securities available-for-sale	346,482	152,262
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	276,583	31,703
Cash and cash equivalents, fair value disclosure	276,583	31,703
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Costs	69,883	121,013
Accumulated gross unrealized gain, before tax	77	10
Accumulated gross unrealized loss, before tax	(61)	(464)
Debt securities, available-for-sale	69,899	120,559
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Costs		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 Costs	46,516	70,631
Accumulated gross unrealized gain, before tax	51	6
Accumulated gross unrealized loss, before tax	(29)	(296)
Debt securities, available-for-sale	46,538	70,341
Municipal securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Costs	5,511
Accumulated gross unrealized gain, before tax	17
Accumulated gross unrealized loss, before tax	0
Debt securities, available-for-sale	5,528
Asset backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Costs	13,596	3,385
Accumulated gross unrealized gain, before tax	9	3
Accumulated gross unrealized loss, before tax	(17)	(1)
Debt securities, available-for-sale	13,588	3,387
Mortgage backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Costs		27,063
Accumulated gross unrealized gain, before tax		1
Accumulated gross unrealized loss, before tax		(119)
Debt securities, available-for-sale		26,945
Agency bond | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Costs	4,260	4,183
Accumulated gross unrealized gain, before tax	0	0
Accumulated gross unrealized loss, before tax	(15)	(35)
Debt securities, available-for-sale	4,245	4,148
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	25,364	16,499
Cash and cash equivalents, fair value disclosure	25,364	16,499
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	251,219	15,204
Cash and cash equivalents, fair value disclosure	$ 251,219	$ 15,204