Company and Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended				9 Months Ended
	Jul. 03, 2018	Feb. 28, 2018	Jan. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Subsidiary Or Equity Method Investee [Line Items]
Common stock, shares issued						68,561,678		59,894,362
Sale of common stock						$ 117,148	$ 64,194
Accumulated deficit						$ 416,603		$ 326,276
Private Financing Transactions
Subsidiary Or Equity Method Investee [Line Items]
Warrants to purchase common stock outstanding	425,000
Warrants to purchase common stock, share price	$ 18.66
Perceptive | Private Financing Transactions
Subsidiary Or Equity Method Investee [Line Items]
Common stock share price	$ 18.66
Number of shares issued and sold	2,679,528
Gross proceeds from debt and equity financing	$ 100,000
Aggregate net proceeds of debt and equity financing, net of fees and offering expenses	97,100
Sale of common stock	50,000
Senior Secured Loan | Perceptive | Private Financing Transactions
Subsidiary Or Equity Method Investee [Line Items]
Convertible bonds, aggregate principal value	$ 50,000
Debt instrument maturity term	5 years
LIBOR | Senior Secured Loan | Perceptive | Private Financing Transactions
Subsidiary Or Equity Method Investee [Line Items]
Debt variable rate basis floor	2.00%
Basis spread on variable rate	9.00%
Convertible Bonds
Subsidiary Or Equity Method Investee [Line Items]
Convertible bonds, aggregate principal value				$ 7,000	$ 68,000		$ 7,000
Convertible bonds, carrying value					55,800
Common stock discount rate				20.00%
Common Stock
Subsidiary Or Equity Method Investee [Line Items]
Aggregate cash proceeds net of underwriting discounts and commissions and offering costs					64,200
Underwriting discounts and commissions					6,100
Offering costs					$ 5,600
Number of shares issued and sold						7,444,528	6,900,000
Sale of common stock						$ 7	$ 7
Common Stock | Convertible Bonds
Subsidiary Or Equity Method Investee [Line Items]
Shares issued upon conversion				795,455	7,727,273
IPO
Subsidiary Or Equity Method Investee [Line Items]
Common stock, shares issued					6,900,000
IPO | Common Stock
Subsidiary Or Equity Method Investee [Line Items]
Common stock share price					$ 11.00
Underwriters
Subsidiary Or Equity Method Investee [Line Items]
Common stock share price		$ 15.25	$ 15.25
Number of shares issued and sold			4,300,000
Number of days granted to underwriters option to purchase additional shares			30 days
Number of additional common stock shares purchase		465,000	645,000
Net proceeds from stock issuance		$ 68,100
Underwriting discounts and commissions and offering expenses		$ 4,600