FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Sep. 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	$ 141,615
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	3,594
Total fair value of available-for-sale securities in a continuous unrealized loss position	145,209
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	(102)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(2)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(104)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	153,362
Due after one year through four years	121,057
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	153,353
Due after one year through four years	121,032
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	98,190
Total fair value of available-for-sale securities in a continuous unrealized loss position	98,190
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	(88)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(88)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	124,796
Due after one year through four years	18,425
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	124,774
Due after one year through four years	18,386
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	18,895
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	3,594
Total fair value of available-for-sale securities in a continuous unrealized loss position	22,489
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	(8)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(2)
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	56,542
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due after one year through four years	56,550
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	15,033
Total fair value of available-for-sale securities in a continuous unrealized loss position	15,033
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	(4)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(4)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	4,559
Due after one year through four years	45,037
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	4,566
Due after one year through four years	45,044
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	8,444
Total fair value of available-for-sale securities in a continuous unrealized loss position	8,444
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	24,007
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	24,013
Certificates of deposit
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	1,053
Total fair value of available-for-sale securities in a continuous unrealized loss position	1,053
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 after one year through four years	1,053
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due after one year through four years	$ 1,052