Investments (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash equivalents and investments
Gross Unrealized Gains	$ 4,000				$ 4,000				$ 4,000	$ 4,000
Gross Unrealized Losses	(63,000)				(7,000)				(63,000)	(7,000)
Total, Amortized Cost	56,960,000				26,422,000				56,960,000	26,422,000
Total, Estimated Fair Value	56,901,000				26,419,000				56,901,000	26,419,000
Other income (expense), net	57,000	29,000	17,000	16,000	46,000	169,000	4,000	(25,000)	119,000	194,000	(84,000)
Reclassification out of accumulated other comprehensive income (loss)
Cash equivalents and investments
Other income (expense), net									0
Short-term investments. | U.S. treasury notes
Cash equivalents and investments
Investments, Amortized Cost	9,020,000				2,071,000				9,020,000	2,071,000
Gross Unrealized Gains	4,000								4,000
Investments, Estimated Fair Value	9,024,000				2,071,000				9,024,000	2,071,000
Short-term investments. | U.S. government agency securities
Cash equivalents and investments
Investments, Amortized Cost	8,853,000				1,454,000				8,853,000	1,454,000
Gross Unrealized Losses	(8,000)								(8,000)
Investments, Estimated Fair Value	8,845,000				1,454,000				8,845,000	1,454,000
Long-term investments | U.S. treasury notes
Cash equivalents and investments
Investments, Amortized Cost	2,494,000				9,116,000				2,494,000	9,116,000
Gross Unrealized Gains					3,000					3,000
Gross Unrealized Losses	(5,000)				(2,000)				(5,000)	(2,000)
Investments, Estimated Fair Value	2,489,000				9,117,000				2,489,000	9,117,000
Long-term investments | U.S. government agency securities
Cash equivalents and investments
Investments, Amortized Cost	31,454,000				8,187,000				31,454,000	8,187,000
Gross Unrealized Gains					1,000					1,000
Gross Unrealized Losses	(50,000)				(5,000)				(50,000)	(5,000)
Investments, Estimated Fair Value	31,404,000				8,183,000				31,404,000	8,183,000
Money market funds
Cash equivalents and investments
Cash equivalents, Estimated Fair Value	357,000				5,356,000				357,000	5,356,000
Certificates of deposit | Short-term investments.
Cash equivalents and investments
Investments, Amortized Cost	2,934,000								2,934,000
Investments, Estimated Fair Value	2,934,000								2,934,000
Certificates of deposit | Long-term investments
Cash equivalents and investments
Investments, Amortized Cost	1,848,000				238,000				1,848,000	238,000
Investments, Estimated Fair Value	$ 1,848,000				$ 238,000				$ 1,848,000	$ 238,000