Financial Instruments - Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable Securities	$ 157,755	$ 51,750
Fair Value Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	193,375	73,088
Cash and Cash Equivalents, Fair Value Disclosure	35,620	21,338
Marketable Securities	157,755	51,750
Fair Value Measurements, Recurring | Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	88,441	31,761
Fair Value Measurements, Recurring | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	36,732	5,992
Fair Value Measurements, Recurring | Agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	20,397	1,998
Fair Value Measurements, Recurring | Asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	19,032	7,998
Fair Value Measurements, Recurring | U.S. treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	14,748	4,001
Fair Value Measurements, Recurring | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	14,025	21,338
Fair Value Measurements, Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	14,025	21,338
Fair Value Measurements, Recurring | Level 1 | Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	0	0
Fair Value Measurements, Recurring | Level 1 | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	0	0
Fair Value Measurements, Recurring | Level 1 | Agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	0	0
Fair Value Measurements, Recurring | Level 1 | Asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	0	0
Fair Value Measurements, Recurring | Level 1 | U.S. treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	0	0
Fair Value Measurements, Recurring | Level 1 | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	14,025	21,338
Fair Value Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	179,350	51,750
Fair Value Measurements, Recurring | Level 2 | Corporate bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	88,441	31,761
Fair Value Measurements, Recurring | Level 2 | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	36,732	5,992
Fair Value Measurements, Recurring | Level 2 | Agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	20,397	1,998
Fair Value Measurements, Recurring | Level 2 | Asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	19,032	7,998
Fair Value Measurements, Recurring | Level 2 | U.S. treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	14,748	4,001
Fair Value Measurements, Recurring | Level 2 | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total fair value of financial assets	$ 0	$ 0