INVESTMENTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 51,213		$ 51,213		$ 7,059
Gross Unrealized Gain	0		0		0
Gross Unrealized (Loss)	(52)		(52)		(20)
Fair Value	51,161		51,161		7,039
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	67,667		67,667		66,693
Gross Unrecognized Gain	2		2		1
Gross Unrecognized (Loss)	(179)		(179)		(191)
Fair Value	67,490		67,490		66,503
Unrealized loss position, fair value [Abstract]
Less than 12 Months	93,485		93,485		51,135
12 Months or greater	16,935		16,935		16,108
Total	110,420		110,420		67,243
Unrealized loss position, unrealized loss [Abstract]
Less than 12 Months	(172)		(172)		(198)
12 Months or greater	(59)		(59)		(13)
Total	(231)		(231)		(211)
Available-for-sale securities and Held-to-maturity securities [Abstract]
Other-than-temporary impairment	0	$ 0	0	$ 0	0
Unrealized loss on available-for-sale investments, net of related income taxes	(32)		(32)		(12)
Redemptions of investments with callable provisions	0	$ 0	0	$ 0
Available-for-sale investments by stated maturity, amortized cost [Abstract]
Amortized Cost, Due in less than one year	45,668		45,668		6,381
Amortized Cost, Due in 1 to 3 years	5,545		5,545		678
Amortized Cost	51,213		51,213		7,059
Available-for-sale investments by stated maturity, fair value [Abstract]
Fair Value, Due in less than one year	45,638		45,638		6,377
Fair Value, Due in 1 to 3 years	5,523		5,523		662
Fair Value, Total investments	51,161		51,161		7,039
Held-to-maturity investments by stated maturity, amortized cost [Abstract]
Amortized Cost, Due in less than one year	44,239		44,239		45,184
Amortized Cost, Due in 1 to 3 years	23,428		23,428		21,509
Amortized Cost, Total investments	67,667		67,667		66,693
Held-to-maturity investments by stated maturity, fair value [Abstract]
Fair Value, Due in less than one year	44,172		44,172		45,124
Fair Value, Due in 1 to 3 years	23,318		23,318		21,379
Fair Value, Total investments	67,490		67,490		66,503
Certificates of Deposit [Member]
Unrealized loss position, fair value [Abstract]
Less than 12 Months	11,152		11,152		2,738
12 Months or greater	0		0		0
Total	11,152		11,152		2,738
Unrealized loss position, unrealized loss [Abstract]
Less than 12 Months	(10)		(10)		(1)
12 Months or greater	0		0		0
Total	(10)		(10)		(1)
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,472		7,472
Gross Unrealized Gain	0		0
Gross Unrealized (Loss)	(1)		(1)
Fair Value	7,471		7,471
Unrealized loss position, fair value [Abstract]
Less than 12 Months	12,457		12,457		2,989
12 Months or greater	0		0		0
Total	12,457		12,457		2,989
Unrealized loss position, unrealized loss [Abstract]
Less than 12 Months	(3)		(3)		(3)
12 Months or greater	0		0		0
Total	(3)		(3)		(3)
Available-for-sale investments by stated maturity, amortized cost [Abstract]
Amortized Cost	7,472		7,472
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,075		43,075		6,381
Gross Unrealized Gain	0		0		0
Gross Unrealized (Loss)	(50)		(50)		(4)
Fair Value	43,025		43,025		6,377
Unrealized loss position, fair value [Abstract]
Less than 12 Months	68,858		68,858		44,387
12 Months or greater	16,935		16,935		16,108
Total	85,793		85,793		60,495
Unrealized loss position, unrealized loss [Abstract]
Less than 12 Months	(157)		(157)		(177)
12 Months or greater	(59)		(59)		(13)
Total	(216)		(216)		(190)
Available-for-sale investments by stated maturity, amortized cost [Abstract]
Amortized Cost	43,075		43,075		6,381
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	666		666		678
Gross Unrealized Gain	0		0		0
Gross Unrealized (Loss)	(1)		(1)		(16)
Fair Value	665		665		662
Unrealized loss position, fair value [Abstract]
Less than 12 Months	1,018		1,018		1,021
12 Months or greater	0		0		0
Total	1,018		1,018		1,021
Unrealized loss position, unrealized loss [Abstract]
Less than 12 Months	(2)		(2)		(17)
12 Months or greater	0		0		0
Total	(2)		(2)		(17)
Available-for-sale investments by stated maturity, amortized cost [Abstract]
Amortized Cost	666		666		678
Certificates of Deposit [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,890		15,890		3,735
Gross Unrecognized Gain	1		1		1
Gross Unrecognized (Loss)	(10)		(10)		(1)
Fair Value	15,881		15,881		3,735
Held-to-maturity investments by stated maturity, fair value [Abstract]
Fair Value, Total investments	15,881		15,881		3,735
Commercial Paper [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,988		4,988		5,588
Gross Unrecognized Gain	0		0		0
Gross Unrecognized (Loss)	(2)		(2)		(3)
Fair Value	4,986		4,986		5,585
Held-to-maturity investments by stated maturity, fair value [Abstract]
Fair Value, Total investments	4,986		4,986		5,585
Corporate Bonds [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	46,435		46,435		57,009
Gross Unrecognized Gain	1		1		0
Gross Unrecognized (Loss)	(166)		(166)		(186)
Fair Value	46,270		46,270		56,823
Held-to-maturity investments by stated maturity, fair value [Abstract]
Fair Value, Total investments	46,270		46,270		56,823
Municipal Bonds [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	354		354		361
Gross Unrecognized Gain	0		0		0
Gross Unrecognized (Loss)	(1)		(1)		(1)
Fair Value	353		353		360
Held-to-maturity investments by stated maturity, fair value [Abstract]
Fair Value, Total investments	$ 353		$ 353		$ 360