FAIR VALUE - Unrealized loss positions (Details) $ in Thousands	Mar. 31, 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	$ 178,837
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	1,436
Total fair value of available-for-sale securities in a continuous unrealized loss position	180,273
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	(236)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(1)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(237)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	156,243
Due after one year through four years	79,440
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	156,137
Due after one year through four years	79,327
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	84,634
Total fair value of available-for-sale securities in a continuous unrealized loss position	84,634
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	109,922
Due after one year through four years	541
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	109,855
Due after one year through four years	542
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	44,740
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	1,436
Total fair value of available-for-sale securities in a continuous unrealized loss position	46,176
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	(34)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(1)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(35)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	378
Due after one year through four years	53,896
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	377
Due after one year through four years	53,864
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	49,463
Total fair value of available-for-sale securities in a continuous unrealized loss position	49,463
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	(123)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(123)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	24,583
Due after one year through four years	25,003
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	24,542
Due after one year through four years	24,921
Commercial paper.
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	17,108
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	17,110
Certificates of deposit
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	4,252
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	$ 4,253