FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Sep. 30, 2018 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	$ 795,705
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	61,850
Total fair value of available-for-sale securities in a continuous unrealized loss position	857,555
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	(1,291)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(188)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(1,479)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	621,947
Due after one year through four years	402,922
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	621,126
Due after one year through four years	402,386
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	258,664
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	24,871
Total fair value of available-for-sale securities in a continuous unrealized loss position	283,535
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	(382)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(91)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(473)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	318,313
Due after one year through four years	97,515
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	317,926
Due after one year through four years	97,533
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	250,157
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	14,975
Total fair value of available-for-sale securities in a continuous unrealized loss position	265,132
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	(410)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(21)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(431)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	186,406
Due after one year through four years	79,157
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	186,135
Due after one year through four years	78,996
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	246,362
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	22,004
Total fair value of available-for-sale securities in a continuous unrealized loss position	268,366
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	(472)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(76)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(548)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	65,931
Due after one year through four years	222,355
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	65,792
Due after one year through four years	221,950
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	28,446
Total fair value of available-for-sale securities in a continuous unrealized loss position	28,446
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	(22)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(22)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	39,216
Due after one year through four years	3,895
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	39,197
Due after one year through four years	3,907
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	12,076
Total fair value of available-for-sale securities in a continuous unrealized loss position	12,076
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	(5)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(5)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	12,081
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	$ 12,076