FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Sep. 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	$ 295,694
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	14,717
Total fair value of available-for-sale securities in a continuous unrealized loss position	310,411
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	(230)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(10)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(240)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	237,317
Due after one year through four years	174,574
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	237,201
Due after one year through four years	174,483
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	131,782
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	14,717
Total fair value of available-for-sale securities in a continuous unrealized loss position	146,499
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	(70)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(10)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(80)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	138,989
Due after one year through four years	74,069
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	138,964
Due after one year through four years	74,041
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	93,585
Total fair value of available-for-sale securities in a continuous unrealized loss position	93,585
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	(56)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(56)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	17,617
Due after one year through four years	83,550
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	17,612
Due after one year through four years	83,501
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	64,869
Total fair value of available-for-sale securities in a continuous unrealized loss position	64,869
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	(103)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(103)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	9,801
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	9,802
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	5,458
Total fair value of available-for-sale securities in a continuous unrealized loss position	5,458
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	20,894
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	20,895
Certificates of deposit
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	50,016
Due after one year through four years	14,956
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	49,928
Due after one year through four years	$ 14,941