FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Mar. 31, 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	$ 843,087
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	26,395
Total fair value of available-for-sale securities in a continuous unrealized loss position	869,482
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	(2,251)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(48)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(2,299)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	529,218
Due after one year through four years	366,551
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	528,072
Due after one year through four years	365,402
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	328,935
Total fair value of available-for-sale securities in a continuous unrealized loss position	328,935
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	(1,073)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(1,073)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	224,976
Due after one year through four years	121,217
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	224,287
Due after one year through four years	120,836
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	252,105
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	1,442
Total fair value of available-for-sale securities in a continuous unrealized loss position	253,547
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	(841)
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	(843)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	30,844
Due after one year through four years	223,546
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	30,753
Due after one year through four years	222,794
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	169,071
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	24,953
Total fair value of available-for-sale securities in a continuous unrealized loss position	194,024
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	(237)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(46)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(283)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	174,495
Due after one year through four years	19,812
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	174,218
Due after one year through four years	19,806
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	51,425
Total fair value of available-for-sale securities in a continuous unrealized loss position	51,425
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	(21)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(21)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	52,015
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	51,994
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	41,551
Total fair value of available-for-sale securities in a continuous unrealized loss position	41,551
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	(79)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(79)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	46,888
Due after one year through four years	1,976
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	46,820
Due after one year through four years	$ 1,966