FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Jun. 30, 2017 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	$ 287,947
Total fair value of available-for-sale securities in a continuous unrealized loss position	287,947
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	(274)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(274)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	144,782
Due after one year through four years	159,331
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	144,642
Due after one year through four years	159,211
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	107,621
Total fair value of available-for-sale securities in a continuous unrealized loss position	107,621
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	(71)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(71)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	97,735
Due after one year through four years	37,734
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	97,681
Due after one year through four years	37,728
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	92,296
Total fair value of available-for-sale securities in a continuous unrealized loss position	92,296
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	(58)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(58)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	38
Due after one year through four years	94,667
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	38
Due after one year through four years	94,611
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	62,820
Total fair value of available-for-sale securities in a continuous unrealized loss position	62,820
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	(144)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(144)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	1,801
Due after one year through four years	1,999
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	1,801
Due after one year through four years	2,000
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	25,210
Total fair value of available-for-sale securities in a continuous unrealized loss position	25,210
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	7,175
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	7,174
Certificates of deposit
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	38,033
Due after one year through four years	24,931
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	37,948
Due after one year through four years	$ 24,872