Investments (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash equivalents and investments
Gross Unrealized Gains					$ 4,000					$ 4,000
Gross Unrealized Losses	$ (482,000)				(63,000)				$ (482,000)	(63,000)
Total, Amortized Cost	247,483,000				56,960,000				247,483,000	56,960,000
Total, Estimated Fair Value	247,001,000				56,901,000				247,001,000	56,901,000
Amortization of premiums and discounts on investments									(647,000)	(202,000)	$ (47,000)
Other income (expense), net	233,000	$ 248,000	$ 202,000	$ 93,000	57,000	$ 29,000	$ 17,000	$ 16,000	776,000	119,000	194,000
Interest Income
Cash equivalents and investments
Interest income on available for sale									1,500,000	400,000	100,000
Other Expense
Cash equivalents and investments
Amortization of premiums and discounts on investments									600,000	200,000
Other Expense | Maximum
Cash equivalents and investments
Amortization of premiums and discounts on investments											$ 100,000
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	12,620,000				9,020,000				12,620,000	9,020,000
Gross Unrealized Gains					4,000					4,000
Gross Unrealized Losses	(14,000)								(14,000)
Investments, Estimated Fair Value	12,606,000				9,024,000				12,606,000	9,024,000
Short-term investments. | U.S. government agency securities
Cash equivalents and investments
Investments, Amortized Cost	102,683,000				8,853,000				102,683,000	8,853,000
Gross Unrealized Losses	(110,000)				(8,000)				(110,000)	(8,000)
Investments, Estimated Fair Value	102,573,000				8,845,000				102,573,000	8,845,000
Long-term investments | U.S. treasury notes
Cash equivalents and investments
Investments, Amortized Cost	22,553,000				2,494,000				22,553,000	2,494,000
Gross Unrealized Losses	(105,000)				(5,000)				(105,000)	(5,000)
Investments, Estimated Fair Value	22,448,000				2,489,000				22,448,000	2,489,000
Long-term investments | U.S. government agency securities
Cash equivalents and investments
Investments, Amortized Cost	45,793,000				31,454,000				45,793,000	31,454,000
Gross Unrealized Losses	(183,000)				(50,000)				(183,000)	(50,000)
Investments, Estimated Fair Value	45,610,000				31,404,000				45,610,000	31,404,000
Money market funds
Cash equivalents and investments
Cash equivalents, Estimated Fair Value	31,761,000				357,000				31,761,000	357,000
Certificates of deposit | Short-term investments.
Cash equivalents and investments
Investments, Amortized Cost	19,774,000				2,934,000				19,774,000	2,934,000
Gross Unrealized Losses	(28,000)								(28,000)
Investments, Estimated Fair Value	19,746,000				2,934,000				19,746,000	2,934,000
Certificates of deposit | Long-term investments
Cash equivalents and investments
Investments, Amortized Cost	12,299,000				1,848,000				12,299,000	1,848,000
Gross Unrealized Losses	(42,000)								(42,000)
Investments, Estimated Fair Value	$ 12,257,000				$ 1,848,000				$ 12,257,000	$ 1,848,000