FAIR VALUE MEASUREMENTS - Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 269,025	$ 276,583
Accumulated gross unrealized gain, before tax	147	77
Accumulated gross unrealized loss, before tax	(8)	(61)
Debt securities, available-for-sale	49,153
Cash, cash equivalents and debt securities available-for-sale, amortized cost	318,039	346,466
Cash, cash equivalents and debt securities available-for-sale	318,178	346,482
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		276,583
Cash and cash equivalents, fair value		276,583
Accumulated gross unrealized gain, before tax	2
Accumulated gross unrealized loss, before tax	0
Cash, cash equivalents and debt securities available-for-sale, amortized cost	267,348
Cash, cash equivalents and debt securities available-for-sale	267,350
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	50,691	69,883
Accumulated gross unrealized gain, before tax	145	77
Accumulated gross unrealized loss, before tax	(8)	(61)
Debt securities, available-for-sale	50,828	69,899
Treasury securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	2,910
Accumulated gross unrealized gain, before tax	2
Accumulated gross unrealized loss, before tax	0
Debt securities, available-for-sale	2,912
Corporate bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	40,128	46,516
Accumulated gross unrealized gain, before tax	122	51
Accumulated gross unrealized loss, before tax	(3)	(29)
Debt securities, available-for-sale	40,247	46,538
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	6,176
Accumulated gross unrealized gain, before tax	0
Accumulated gross unrealized loss, before tax	(2)
Debt securities, available-for-sale	6,174
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	2,098	5,511
Accumulated gross unrealized gain, before tax	23	17
Accumulated gross unrealized loss, before tax	0	0
Debt securities, available-for-sale	2,121	5,528
Asset backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost		13,596
Accumulated gross unrealized gain, before tax		9
Accumulated gross unrealized loss, before tax		(17)
Debt securities, available-for-sale		13,588
Agency bond | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	2,289	4,260
Accumulated gross unrealized gain, before tax	0	0
Accumulated gross unrealized loss, before tax	(3)	(15)
Debt securities, available-for-sale	2,286	4,245
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	23,533	25,364
Cash and cash equivalents, fair value	23,533	25,364
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	240,905	251,219
Cash and cash equivalents, fair value	240,905	251,219
Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		276,583
Cash, cash equivalents and debt securities available-for-sale	269,025
Cash and Cash Equivalents | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		276,583
Cash, cash equivalents and debt securities available-for-sale	264,438
Cash and Cash Equivalents | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	4,587
Cash and Cash Equivalents | Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	4,587
Cash and Cash Equivalents | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		25,364
Cash and cash equivalents, fair value	23,533
Cash and Cash Equivalents | Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		251,219
Cash and cash equivalents, fair value	240,905
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale		69,899
Cash, cash equivalents and debt securities available-for-sale	27,486
Short-term Investments | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	27,486	69,899
Short-term Investments | Corporate bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	23,613	46,538
Short-term Investments | Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	1,587
Short-term Investments | Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0	5,528
Short-term Investments | Asset backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale		13,588
Short-term Investments | Agency bond | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	2,286	$ 4,245
Long-term Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, cash equivalents and debt securities available-for-sale	21,667
Long-term Debt | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	2,912
Long-term Debt | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	18,755
Long-term Debt | Treasury securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	2,912
Long-term Debt | Corporate bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	16,634
Long-term Debt | Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	0
Long-term Debt | Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	2,121
Long-term Debt | Agency bond | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available-for-sale	$ 0