CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 173,206	$ 159,951
Unrealized Gains (Losses)	(46)	13
Fair Value	173,160	159,964
U.S. government and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	160,086	145,345
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5,099	5,095
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,975	9,524
Short-term marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112,868	101,388
Unrealized Gains (Losses)	(11)	4
Fair Value	112,857	101,392
Short-term marketable securities | U.S. government and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	99,792	88,651
Unrealized Gains (Losses)	(9)	3
Fair Value	99,783	88,654
Short-term marketable securities | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,099	5,093
Unrealized Gains (Losses)	0	2
Fair Value	5,099	5,095
Short-term marketable securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,977	7,644
Unrealized Gains (Losses)	(2)	(1)
Fair Value	7,975	7,643
Long-term marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,338	58,563
Unrealized Gains (Losses)	(35)	9
Fair Value	60,303	58,572
Long-term marketable securities | U.S. government and municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,338	56,683
Unrealized Gains (Losses)	(35)	8
Fair Value	$ 60,303	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