SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investments in an Unrealized Loss Position (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
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	$ 246,069,000	$ 152,674,000
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	3,887,000
Total fair value of available-for-sale securities in a continuous unrealized loss position	249,956,000	152,674,000
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	(444,000)	(160,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(2,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(446,000)	(160,000)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	120,519,000
Due after one year through two years	145,669,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	120,421,000
Due after one year through two years	145,323,000
Concentration of Credit Risk
Cash and cash equivalents, federal government agency insured limit	250,000
Cash and cash equivalents in excess of federal government agency insured limit	40,100,000
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	166,238,000	113,960,000
Total fair value of available-for-sale securities in a continuous unrealized loss position	166,238,000	113,960,000
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	(262,000)	(135,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(262,000)	(135,000)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	115,647,000
Due after one year through two years	63,824,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	115,555,000
Due after one year through two years	63,656,000
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	72,792,000	33,073,000
Total fair value of available for sale securities in a continuous unrealized loss position for greater than twelve months	3,887,000
Total fair value of available-for-sale securities in a continuous unrealized loss position	76,679,000	33,073,000
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	(164,000)	(18,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position for greater than twelve months	(2,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(166,000)	(18,000)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	373,000
Due after one year through two years	77,288,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	373,000
Due after one year through two years	77,122,000
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	7,039,000
Total fair value of available-for-sale securities in a continuous unrealized loss position	7,039,000
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	(18,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position	(18,000)
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	2,500,000
Due after one year through two years	4,557,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	2,494,000
Due after one year through two years	4,545,000
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		5,641,000
Total fair value of available-for-sale securities in a continuous unrealized loss position		5,641,000
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		(7,000)
Total gross unrealized losses of available-for-sale securities in a continuous unrealized loss position		$ (7,000)
Certificates of deposit
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	1,999,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	$ 1,999,000