FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Fair value of investments in unrealized loss positions
Total fair value of available-for-sale securities in a continuous unrealized loss position for less than twelve months	$ 89,698
Total fair value of available-for-sale securities in a continuous unrealized loss position	89,698
Gross unrealized loss of investments in unrealized loss positions
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for less than twelve months	(86)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(86)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	109,611
Due after one year through four years	100,801
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	109,635
Due after one year through four years	100,806
Corporate bonds
Fair value of investments in unrealized loss positions
Total fair value of available-for-sale securities in a continuous unrealized loss position for less than twelve months	41,402
Total fair value of available-for-sale securities in a continuous unrealized loss position	41,402
Gross unrealized loss of investments in unrealized loss positions
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for less than twelve months	(41)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(41)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	98,989
Due after one year through four years	33,865
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	99,012
Due after one year through four years	33,894
Asset backed securities
Fair value of investments in unrealized loss positions
Total fair value of available-for-sale securities in a continuous unrealized loss position for less than twelve months	43,177
Total fair value of available-for-sale securities in a continuous unrealized loss position	43,177
Gross unrealized loss of investments in unrealized loss positions
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for less than twelve months	(44)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(44)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	1
Due after one year through four years	62,379
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	1
Due after one year through four years	62,347
U.S. government agency securities
Fair value of investments in unrealized loss positions
Total fair value of available-for-sale securities in a continuous unrealized loss position for less than twelve months	2,499
Total fair value of available-for-sale securities in a continuous unrealized loss position	2,499
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	2,500
Due after one year through four years	4,557
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	2,500
Due after one year through four years	4,565
Commercial Paper.
Fair value of investments in unrealized loss positions
Total fair value of available-for-sale securities in a continuous unrealized loss position for less than twelve months	2,620
Total fair value of available-for-sale securities in a continuous unrealized loss position	2,620
Gross unrealized loss of investments in unrealized loss positions
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for less than twelve months	(1)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(1)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	2,621
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	2,620
Certificates of deposit
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	5,500
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	$ 5,502