Investments Summary (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	$ 209,913,000	$ 184,754,000
Cash equivalents and available-for-sale investments, gains	21,000	42,000
Cash equivalents and available-for-sale securities, losses	91,000	72,000
Cash equivalents and available-for-sale investments, estimated fair value	209,843,000	184,724,000
Gross Amortized Cost and Estimated Fair Value of Investments
Due in one year or less, gross amortized cost	154,192,000	153,346,000
Due in one year or less, fair value	154,124,000	153,328,000
Due in one year through five years, gross amortized cost	55,721,000	31,408,000
Due after one year through five years, fair value	55,719,000	31,396,000
Total, amortized cost basis	209,913,000	184,754,000
Total, fair value	209,843,000	184,724,000
Commercial papers
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	17,847,000	9,996,000
Cash equivalents and available-for-sale investments, estimated fair value	17,847,000	9,996,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	17,847,000	9,996,000
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	5,087,000	6,706,000
Cash equivalents and available-for-sale investments, estimated fair value	5,087,000	6,706,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	5,087,000	6,706,000
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	0	1,441,000
Cash equivalents and available-for-sale investments, estimated fair value	0	1,440,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	0	1,440,000
U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	4,028,000	3,908,000
Cash equivalents and available-for-sale investments, estimated fair value	4,030,000	3,913,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	4,030,000	3,913,000
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	182,951,000	162,703,000
Cash equivalents and available-for-sale investments, estimated fair value	182,879,000	162,669,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	182,879,000	162,669,000
Accumulated other comprehensive income gains losses | Commercial papers
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Accumulated other comprehensive income gains losses | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Accumulated other comprehensive income gains losses | U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	1,000
Accumulated other comprehensive income gains losses | U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, gains	2,000	5,000
Cash equivalents and available-for-sale securities, losses	0	0
Accumulated other comprehensive income gains losses | Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, gains	19,000	37,000
Cash equivalents and available-for-sale securities, losses	91,000	71,000
Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	20,935,000	15,557,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	20,935,000	15,557,000
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	188,908,000	169,167,000
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	$ 188,908,000	$ 169,167,000