CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 135,775	$ 159,951
Unrealized Gains (Losses)	71	13
Fair Value	135,846	159,964
U.S. government and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	126,240	145,345
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value	2,990	5,095
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	6,616	9,524
Short-term marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	103,111	101,388
Unrealized Gains (Losses)	27	4
Fair Value	103,138	101,392
Short-term marketable securities | U.S. government and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93,507	88,651
Unrealized Gains (Losses)	25	3
Fair Value	93,532	88,654
Short-term marketable securities | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,990	5,093
Unrealized Gains (Losses)	0	2
Fair Value	2,990	5,095
Short-term marketable securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,614	7,644
Unrealized Gains (Losses)	2	(1)
Fair Value	6,616	7,643
Long-term marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,664	58,563
Unrealized Gains (Losses)	44	9
Fair Value	32,708	58,572
Long-term marketable securities | U.S. government and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,664	56,683
Unrealized Gains (Losses)	44	8
Fair Value	$ 32,708	56,691
Long-term marketable securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,880
Unrealized Gains (Losses)		1
Fair Value		$ 1,881