FAIR VALUE MEASUREMENTS (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	$ 588,172,000	$ 335,965,000
U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	42,229,000	27,205,000
U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	161,857,000	115,466,000
U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	103,809,000	91,566,000
U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	133,818,000	23,082,000
Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	146,459,000	78,646,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	388,170,000	203,292,000
Fair value measurements transfer to Level 1 From Level 2	6,600,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	42,229,000	27,205,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	114,180,000	56,969,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	85,302,000	40,472,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	146,459,000	78,646,000
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	200,002,000	132,673,000
Fair value measured transfer to Level 2 From Level 1	8,500,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	47,677,000	58,497,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	18,507,000	51,094,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	133,818,000	23,082,000
Recurring basis | Total
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	588,172,000	335,965,000
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	42,229,000	27,205,000
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	161,857,000	115,466,000
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	103,809,000	91,566,000
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	133,818,000	23,082,000
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	146,459,000	78,646,000
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Convertible Subordinated Notes Due 2015
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes		194,050,000
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	476,531,000
Nonrecurring basis | Total | Convertible Subordinated Notes Due 2015
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes		194,050,000
Nonrecurring basis | Total | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	$ 476,531,000