Financial Instruments - Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	$ 73,474	$ 73,088
Included in marketable securities	54,918	51,750
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	18,556	21,338
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	54,918	51,750
Corporate securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	29,696	31,761
Corporate securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	29,696	31,761
Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	18,556	21,338
Money market funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	18,556	21,338
Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	10,025	7,998
Asset-backed Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	10,025	7,998
U.S. treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	9,211	4,001
U.S. treasury securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	9,211	4,001
Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	3,985	5,992
Commercial paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	3,985	5,992
Agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	2,001	1,998
Agency securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	2,001	1,998
Fair Value Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Included in cash and cash equivalents	18,556	21,338
Included in marketable securities	$ 54,918	$ 51,750