Investments Summary (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	$ 246,514,000	$ 251,853,000
Cash equivalents and available-for-sale investments, gains	35,000	82,000
Cash equivalents and available-for-sale securities, losses	70,000	57,000
Cash equivalents and available-for-sale investments, estimated fair value	246,479,000	251,878,000
Due in one year or less, gross amortized cost	223,456,000
Due in one year or less, fair value	223,413,000
Due in one year through five years, gross amortized cost	23,059,000
Due after one year through five years, fair value	23,066,000
Total, amortized cost basis	246,515,000
Total, fair value	246,479,000	251,878,000
Cash Equivalents and Short-term Investments [Member] | Commercial papers
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	3,199,000	19,996,000
Cash equivalents and available-for-sale investments, estimated fair value	3,199,000	19,996,000
Total, fair value	3,199,000	19,996,000
Cash Equivalents and Short-term Investments [Member] | Accumulated other comprehensive 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
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	42,994,000	46,338,000
Total, fair value	42,994,000	46,338,000
Cash Equivalents [Member] | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	42,994,000	15,107,000
Cash equivalents and available-for-sale investments, estimated fair value	42,994,000	15,107,000
Total, fair value	42,994,000	15,107,000
Cash Equivalents [Member] | Accumulated other comprehensive 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
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	203,485,000	205,540,000
Total, fair value	203,485,000	205,540,000
Short-term Investments [Member] | U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, amortized cost	6,067,000	6,040,000
Cash equivalents and available-for-sale investments, estimated fair value	6,072,000	6,045,000
Total, fair value	6,072,000	6,045,000
Short-term Investments [Member] | 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	194,254,000	210,710,000
Cash equivalents and available-for-sale investments, estimated fair value	194,214,000	210,730,000
Total, fair value	194,214,000	210,730,000
Short-term Investments [Member] | Accumulated other comprehensive gains losses | U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and available-for-sale investments, gains	5,000	5,000
Cash equivalents and available-for-sale securities, losses	0	0
Short-term Investments [Member] | Accumulated other comprehensive 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	30,000	77,000
Cash equivalents and available-for-sale securities, losses	$ 70,000	$ 57,000