Fair Value Measurements (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	$ 425,472,000	$ 502,831,000
U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	37,116,000	42,158,000
U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	110,230,000	141,331,000
U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	88,645,000	94,977,000
U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	74,487,000	102,022,000
Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	114,994,000	122,343,000
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	378,989,000	327,827,000
Fair value measurements transfer to Level 1 From Level 2	2,800,000	2,900,000
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	37,116,000	42,158,000
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	82,895,000	98,236,000
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	69,497,000	61,591,000
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	74,487,000	3,499,000
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	114,994,000	122,343,000
Recurring 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	0	0
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	46,483,000	175,004,000
Fair value measured transfer to Level 2 From Level 1	1,000,000	1,000,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
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	27,335,000	43,095,000
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	19,148,000	33,386,000
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	0	98,523,000
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	0	0
Recurring basis | Total
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	425,472,000	502,831,000
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	37,116,000	42,158,000
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	110,230,000	141,331,000
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	88,645,000	94,977,000
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	74,487,000	102,022,000
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	114,994,000	122,343,000
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	369,685,000	408,250,000
Nonrecurring basis | Total | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Total convertible subordinated notes	$ 369,685,000	$ 408,250,000