Summary of Significant Accounting Policies - Additional Information (Detail)						1 Months Ended
	Nov. 17, 2017 USD ($) $ / shares shares	Sep. 14, 2016 USD ($) $ / shares shares	Jan. 12, 2016 USD ($) $ / shares shares	Apr. 20, 2015 USD ($) $ / shares shares	Jul. 23, 2014 USD ($) $ / shares shares	May 31, 2017 ft²	May 31, 2016 ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								$ 849,000	$ 564,000
Issuance of common stock, shares | shares	4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share | $ / shares	$ 85.00	$ 39.75	$ 47.50	$ 52.50
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering costs	$ 325,800,000	$ 189,200,000	$ 140,400,000	$ 129,100,000
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, shares | shares					5,750,000
Common stock price per share | $ / shares					$ 18.00
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts					$ 94,000,000
Operating Lease Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								39,000	39,000
Operating Lease Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								300,000
Operating Lease One [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit								$ 500,000	$ 500,000
Additional Office space rented under operating lease | ft²						22,067	19,805
Lease expire date						Feb. 28, 2022	Feb. 28, 2022