Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended			24 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2014
Organization And Basis Of Presentation [Line Items]
Increase in current assets	$ 149,712,000				$ 203,992,000				$ 149,712,000	$ 203,992,000		$ 149,712,000
Increase in current liabilities	17,871,000				19,201,000				17,871,000	19,201,000		17,871,000
Decrease in accumulated deficit	(278,214,000)				(213,875,000)				(278,214,000)	(213,875,000)		(278,214,000)
Net loss	10,114,000	$ (20,724,000)	$ (25,721,000)	$ (28,008,000)	(31,276,000)	$ (29,031,000)	$ (28,613,000)	$ (23,467,000)	$ (64,339,000)	(112,387,000)	$ (29,621,000)
Cash and cash equivalents original maturity dates									P90D
Short-term investments maturity period description									Short-term investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than one year, from the date of acquisition.
Impairment loss of long lived assets									$ 0
Income tax provision (benefit)									1,179,000		(29,000)
Unrecognized excess tax benefit	20,734,000				$ 3,892,000				$ 20,734,000	$ 3,892,000	$ 3,298,000	20,734,000	$ 2,815,000
Accounts Receivable
Provision for doubtful accounts												0
Minimum [Member]
Organization And Basis Of Presentation [Line Items]
Property and equipment, estimated useful lives									3 years
Maximum [Member]
Organization And Basis Of Presentation [Line Items]
Property and equipment, estimated useful lives									5 years
Adjustments | Adoption of new accounting pronouncement
Organization And Basis Of Presentation [Line Items]
Increase in current assets	(5,000,000)								$ (5,000,000)			(5,000,000)
Increase in current liabilities	(1,000,000)								(1,000,000)			(1,000,000)
Decrease in accumulated deficit	$ (4,000,000)								$ (4,000,000)			$ (4,000,000)