Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)	Aug. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policy [Line Items]
Number of operating segments | Segment	1
Realized gains or losses on short-term investment	$ 0	$ 0
Unrealized losses on short-term investment	43,000	11,000
Other than temporary impairment evaluated	0	0
Amortization of deferred financing costs	$ 0	107,000
Deferred public offering costs		1,620,000		$ 1,846,000
Tax benefits recognized likelihood of being realized upon ultimate settlement	greater than 50 percent
Accrued interest or penalties	$ 0	0
Accrued interest or penalties recognized	0	0
Equity Offering [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred public offering costs		2,377,000
2021 Convertible Notes [Member] | Senior Notes [Member]
Summary Of Significant Accounting Policy [Line Items]
Amortization of debt issuance costs	$ 222,000
2020 Convertible Notes Offering [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred public offering costs		$ 627,000
2020 Convertible Notes Offering [Member] | Follow-on Offering [Member]
Summary Of Significant Accounting Policy [Line Items]
Deferred public offering costs			$ 462,000