Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	$ 335,965	$ 235,751
U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	27,205	66,174
U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	115,466	93,175
U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	91,566	2,705
U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	23,082	34,976
Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	78,646	38,721
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	203,292	163,501
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	27,205	66,174
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	56,969	55,901
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	40,472	2,705
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	78,646	38,721
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	132,673	72,250
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	58,497	37,274
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	51,094
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	23,082	34,976
Recurring basis | Significant Other Observable Inputs, Level 2 | Convertible subordinated notes
Estimated fair values of entity's financial assets and liabilities
Total liabilities measured at fair value	194,050	189,588
Recurring basis | Total
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	335,965	235,751
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	27,205	66,174
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	115,466	93,175
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	91,566	2,705
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	23,082	34,976
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	78,646	38,721
Recurring basis | Total | Convertible subordinated notes
Estimated fair values of entity's financial assets and liabilities
Total liabilities measured at fair value	$ 194,050	$ 189,588