FAIR VALUE - Unrealized loss positions and contractual underlying maturities (Details) $ in Thousands	Jun. 30, 2019 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	$ 65,871
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	35,160
Total fair value of available-for-sale securities in a continuous unrealized loss position	101,031
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	(18)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(26)
Contractual maturities of the available-for-sale investments, Cost
Due in one year or less	514,891
Due after one year through four years	516,742
Contractual maturities of the available-for-sale investments, Fair Value
Due in one year or less	515,693
Due after one year through four years	518,671
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	27,195
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	2,500
Total fair value of available-for-sale securities in a continuous unrealized loss position	29,695
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	(3)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(3)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(6)
Contractual maturities of the available-for-sale investments, Cost
Due in one year or less	208,610
Due after one year through four years	238,559
Contractual maturities of the available-for-sale investments, Fair Value
Due in one year or less	208,852
Due after one year through four years	239,679
U.S. government agency securities
Contractual maturities of the available-for-sale investments, Cost
Due in one year or less	164,809
Due after one year through four years	55,002
Contractual maturities of the available-for-sale investments, Fair Value
Due in one year or less	165,153
Due after one year through four years	55,020
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	16,774
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	32,660
Total fair value of available-for-sale securities in a continuous unrealized loss position	49,434
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	(3)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(15)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(18)
Contractual maturities of the available-for-sale investments, Cost
Due in one year or less	77,717
Due after one year through four years	214,256
Contractual maturities of the available-for-sale investments, Fair Value
Due in one year or less	77,862
Due after one year through four years	214,992
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	5,004
Total fair value of available-for-sale securities in a continuous unrealized loss position	5,004
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, Cost
Due in one year or less	46,856
Due after one year through four years	8,925
Contractual maturities of the available-for-sale investments, Fair Value
Due in one year or less	46,928
Due after one year through four years	8,980
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	16,898
Total fair value of available-for-sale securities in a continuous unrealized loss position	16,898
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, Cost
Due in one year or less	16,899
Contractual maturities of the available-for-sale investments, Fair Value
Due in one year or less	$ 16,898