FAIR VALUE MEASUREMENTS (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	$ 526,777,000	$ 335,965,000
Fair value measurements transfer to Level 1 From Level 2	11,900,000
Fair value Level 2 securities		12,000,000
U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	27,116,000	27,205,000
U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	157,929,000	115,466,000
U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	104,071,000	91,566,000
U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	44,967,000	23,082,000
Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	192,694,000	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	430,253,000	203,292,000
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	27,116,000	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	113,583,000	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	91,871,000	40,472,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	4,989,000
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	192,694,000	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	96,524,000	132,673,000
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	44,346,000	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	12,200,000	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	39,978,000	23,082,000
Recurring basis | Total
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	526,777,000	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	27,116,000	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	157,929,000	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	104,071,000	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	44,967,000	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	192,694,000	78,646,000
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	712,516,000
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Convertible Subordinated Notes Due 2015
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	268,596,000
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	443,920,000
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 | Total
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	712,516,000
Nonrecurring basis | Total | Convertible Subordinated Notes Due 2015
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	268,596,000	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	$ 443,920,000