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Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	148 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 29, 2017
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date	Nov. 22, 2005
Accumulated deficit	$ (305,533)	$ (289,577)
Net proceeds from sale of securities		333,300
Total cash, cash equivalents and marketable securities	89,100
Term Loan [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Aggregate principal balance	20,000		$ 20,000
Loan agreement minimum liquidity required	$ 21,000	$ 21,000

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