Summary of Significant Accounting Policies - Additional Information (Detail)							1 Months Ended				3 Months Ended	12 Months Ended
	Feb. 13, 2018 USD ($) $ / shares shares	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	Dec. 31, 2018 USD ($) ft²	Oct. 31, 2018 ft²	May 31, 2018 ft²	Apr. 30, 2018 ft²	Mar. 31, 2018 ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							$ 2,367,000					$ 2,367,000	$ 849,000
Issuance of common stock, shares | shares	4,032,012	4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share | $ / shares	$ 164.00	$ 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	$ 631,200,000	$ 325,800,000	$ 189,200,000	$ 140,400,000	$ 129,100,000
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Lease liabilities																$ 45,000,000
Right-of-use assets																$ 41,000,000
Accounting Standards Update 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents increased by amount due to inclusion of restricted cash												2,400,000	800,000	$ 600,000	$ 39,000
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	39,000
Additional Office space rented under operating lease | ft²										40,419	19,805
Lease expire date										Aug. 31, 2024	Feb. 28, 2022	Aug. 31, 2024
Operating Lease Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							300,000					$ 300,000	300,000
Additional Office space rented under operating lease | ft²									15,525
Lease expire date									Nov. 30, 2024
Operating Lease One [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							500,000					$ 500,000	$ 500,000
Additional Office space rented under operating lease | ft²								58,442				58,442
Lease expire date								Aug. 31, 2024				Aug. 31, 2024
Operating Lease Six [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							400,000					$ 400,000
Operating Lease Four [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							400,000					$ 400,000
Additional Office space rented under operating lease | ft²												15,975
Lease expire date												Feb. 28, 2024
Operating Lease Five [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit							$ 700,000					$ 700,000
Additional Office space rented under operating lease | ft²							15,975
Lease expire date							Feb. 28, 2024