Summary of Significant Accounting Policies - Additional Information (Detail)											1 Months Ended					3 Months Ended	12 Months Ended
	Feb. 28, 2019 ft²	Feb. 27, 2019 USD ($) $ / shares shares	Sep. 30, 2018 ft²	Feb. 13, 2018 USD ($) $ / shares shares	Jan. 01, 2018 ft²	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	Mar. 31, 2019 ft²	Dec. 31, 2018 USD ($) ft²	Oct. 31, 2018 ft²	May 31, 2018 ft²	Apr. 30, 2018 ft²	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit												$ 2,367,000					$ 2,367,000	$ 2,367,000
Lease expire date																	Jan. 31, 2022
Intangible assets												0						0
Issuance of common stock, shares | shares		3,833,334		4,032,012		4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share | $ / shares		$ 150.00		$ 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		$ 560,900,000		$ 631,200,000		$ 325,800,000	$ 189,200,000	$ 140,400,000	$ 129,100,000
Lease liabilities																	$ 37,092,000
ASC Topic 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Lease liabilities																	37,092,000				$ 44,200,000
Right-of-use assets																					$ 41,100,000
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	2,400,000	$ 800,000	$ 600,000
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										$ 94,000,000.0
CyDex License Agreement [Member]
Summary Of Significant Accounting Policies [Line Items]
Milestone payments																$ 3,000,000.0
The Regents of University of California [Member]
Summary Of Significant Accounting Policies [Line Items]
Milestone payments																$ 500,000
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					19,805										40,419
Lease expire date					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		54,943		54,943						63,017		58,442
Lease expire date					Aug. 15, 2024						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	$ 400,000
Additional Office space rented under operating lease | ft²																		15,975
Lease expire date																		Feb. 29, 2024
Operating Lease Five [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit												$ 700,000					$ 700,000	$ 700,000
Additional Office space rented under operating lease | ft²												15,975
Lease expire date												Feb. 29, 2024