Stockholders' Equity - Controlled Equity Offering Sales Agreement - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 01, 2017	Nov. 07, 2016	Dec. 07, 2015
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Net proceeds from issuance of common stock			$ 90,830,000
Common Stock [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Issuance of common stock, net of issuance costs			2,950,000
Controlled Equity Offering Sales Agreement [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Net proceeds from issuance of common stock	$ 74,885,000	$ 50,573,000	$ 1,502,000
Controlled Equity Offering Sales Agreement [Member] | Common Stock [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Issuance of common stock, net of issuance costs	7,308,202	5,645,082	121,314
Net proceeds from issuance of common stock	$ 1,000		$ 1,000
Cantor Fitzgerald & Co. [Member] | Controlled Equity Offering Sales Agreement [Member] | Common Stock [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Issuance of common stock, net of issuance costs	3,405,763	5,645,082	121,314
Net proceeds from issuance of common stock	$ 36,978,000	$ 50,573,000	$ 1,502,000
Maximum [Member] | Cantor Fitzgerald & Co. [Member] | Controlled Equity Offering Sales Agreement [Member]
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]
Aggregate offering price				$ 75,000,000	$ 50,000,000	$ 50,000,000
Percentage of commission of gross proceeds from the sale of Shares	3.00%