FAIR VALUE MEASUREMENTS (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Marketable securities
Available-for-sale securities	$ 138,131,000	$ 94,776,000
Available-for-sale securities in a continuous unrealized loss position
Total unrealized losses of available-for-sale securities in a continuous unrealized loss position for less than twelve months	14,800	4,800
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Due in one year or less	85,452,000
Due after one year through two years	52,679,000
Estimated Fair Value	138,131,000	94,776,000
Contractual maturities of the available-for-sale investments in debt securities, Cost
Due in one year or less	85,409,000
Due after one year through two years	52,604,000
Cost	138,013,000
Total unrealized losses of available for sale securities in a continuous unrealized loss position for greater than twelve months	0	0
Fair Value
Marketable securities
Available-for-sale securities	147,841,000	108,121,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	147,841,000	108,121,000
Fair Value | Cash equivalents
Marketable securities
Cash and money market	9,710,000	13,095,000
Fair Value | U.S. government agency securities
Marketable securities
Available-for-sale securities	44,533,000	44,308,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	44,533,000	44,308,000
Fair Value | Corporate bonds
Marketable securities
Corporate bond		250,000
Available-for-sale securities	81,025,000	43,330,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	81,025,000	43,330,000
Fair Value | Certificates of deposit
Marketable securities
Available-for-sale securities	7,574,000	5,939,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	7,574,000	5,939,000
Fair Value | Commercial paper
Marketable securities
Available-for-sale securities	4,999,000	1,199,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	4,999,000	1,199,000
Quoted Prices in Active Markets for Identical Assets (Level1)
Marketable securities
Cash and money market	9,710,000	13,095,000
Quoted Prices in Active Markets for Identical Assets (Level1) | Cash equivalents
Marketable securities
Cash and money market	9,710,000	13,095,000
Significant Other Observable Inputs (Level 2)
Marketable securities
Long-term debt	1,000,000	1,000,000
Available-for-sale securities	138,131,000	95,026,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	138,131,000	95,026,000
Significant Other Observable Inputs (Level 2) | U.S. government agency securities
Marketable securities
Available-for-sale securities	44,533,000	44,308,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	44,533,000	44,308,000
Significant Other Observable Inputs (Level 2) | Corporate bonds
Marketable securities
Corporate bond		250,000
Available-for-sale securities	81,025,000	43,330,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	81,025,000	43,330,000
Significant Other Observable Inputs (Level 2) | Certificates of deposit
Marketable securities
Available-for-sale securities	7,574,000	5,939,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	7,574,000	5,939,000
Significant Other Observable Inputs (Level 2) | Commercial paper
Marketable securities
Available-for-sale securities	4,999,000	1,199,000
Contractual maturities of the available-for-sale investments in debt securities, Fair Value
Estimated Fair Value	$ 4,999,000	$ 1,199,000