FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Jun. 30, 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	$ 50,946
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	421
Total fair value of available-for-sale securities in a continuous unrealized loss position	51,367
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	(19)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(1)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(20)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	112,851
Due after one year through four years	62,105
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	112,919
Due after one year through four years	62,148
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	19,463
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	421
Total fair value of available-for-sale securities in a continuous unrealized loss position	19,884
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	(9)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(1)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(10)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	105,164
Due after one year through four years	10,715
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	105,224
Due after one year through four years	10,735
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	31,483
Total fair value of available-for-sale securities in a continuous unrealized loss position	31,483
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	(10)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(10)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due after one year through four years	48,394
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due after one year through four years	48,411
U.S. government agency securities
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	1,562
Due after one year through four years	2,996
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	1,568
Due after one year through four years	3,002
Commercial paper.
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	2,625
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	2,626
Certificates of deposit
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	3,500
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	$ 3,501