Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policy [Line Items]
Number of operating segments | Segment	1
Realized gains or losses on short-term investment	$ 0	$ 0
Net unrealized gains (losses) on short-term investment	38,000	(43,000)
Other than temporary impairment evaluated	$ 0	0
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
Share based compensation forfeiture rate prior to adoption	0.00%
2021 Convertible Notes [Member] | Level 2 [Member]
Summary Of Significant Accounting Policy [Line Items]
Short-term investments fair value disclosure	$ 38,610,000	55,640,000
2021 Convertible Notes [Member] | Senior Notes [Member]
Summary Of Significant Accounting Policy [Line Items]
Amortization of debt issuance costs	$ 581,000	$ 222,000