Stockholders' Equity - Additional Information (Detail)								1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended	15 Months Ended
	Sep. 25, 2018 USD ($) $ / shares shares	Jun. 11, 2018 USD ($) $ / shares shares	Feb. 06, 2018 USD ($) $ / shares shares	Sep. 29, 2017 USD ($) $ / shares shares	Sep. 28, 2017 USD ($) shares	Feb. 08, 2017 USD ($) shares	Apr. 13, 2016 $ / shares shares	May 31, 2015 USD ($) $ / shares shares	Feb. 28, 2014 Tranche $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 $ / shares shares	Dec. 31, 2016 USD ($)	Feb. 20, 2014 USD ($)
Equity [Line Items]
Preferred stock, shares authorized										10,000,000		10,000,000		10,000,000
Preferred stock, par value | $ / shares										$ 0.00001		$ 0.00001		$ 0.00001
Preferred stock, shares outstanding										0		0		0
Common stock, shares authorized										300,000,000		300,000,000		300,000,000
Common stock, par value | $ / shares										$ 0.00001		$ 0.00001		$ 0.00001
Number of common stock for issuance in exchange for future services from founders									1,000,000
Common stock price per share | $ / shares									$ 0.01
Number of tranches | Tranche									2
Unrecognized compensation expense | $								$ 62,000								$ 168,000
Common stock shares repurchased								633,810
Repurchase of common stock from stockholders, per share | $ / shares								$ 0.00001
Remaining number of repurchased shares								366,190
Common stock sold							7,500,000			0		0		0
Common stock, price per share | $ / shares							$ 1.25
Cash proceeds from common stock issuance | $													$ 131,402,000
Shares issued under ESPP										9,568	72,597
Common Stock [Member] | February 2018 Offering [Member]
Equity [Line Items]
Common stock sold			12,650,000
Common stock, price per share | $ / shares			$ 5.00
Cash proceeds from common stock issuance | $			$ 58,700,000
Common Stock [Member] | June 2018 Offering [Member]
Equity [Line Items]
Common stock sold		8,625,000
Common stock, price per share | $ / shares		$ 9.00
Cash proceeds from common stock issuance | $		$ 72,300,000
Common Stock [Member] | September 2018 Offering [Member]
Equity [Line Items]
Common stock sold	9,500,000
Common stock, price per share | $ / shares	$ 18.50
Cash proceeds from common stock issuance | $	$ 165,000,000
PoC Capital, LLC [Member]
Equity [Line Items]
Prepaid clinical study costs | $						$ 1,800,000
Amortization expense | $										$ 6,000	$ 252,000	$ 140,000	477,000
PoC Capital, LLC [Member] | Common Stock [Member] | Stock Purchase Agreement [Member]
Equity [Line Items]
Common stock sold						1,286,173
PoC Capital, LLC [Member] | Common Stock [Member] | Stock Purchase Agreement [Member] | Clinical Studies [Member]
Equity [Line Items]
Common stock sold aggregate purchase price | $						$ 1,800,000
LPC [Member] | Common Stock [Member] | Registered Offering Purchase Agreement [Member]
Equity [Line Items]
Common stock sold				701,282
Common stock sold aggregate purchase price | $				$ 1,300,000
Common stock, price per share | $ / shares				$ 1.78
Registration statement commencement date				Jun. 20, 2016
LPC [Member] | Common Stock [Member] | Commitment Purchase Agreement [Member] | Registration Rights Agreement [Member]
Equity [Line Items]
Common stock sold					343,051
Common stock purchase agreement term					30 months
Common stock issued as consideration for commitment					100,000
Gross proceeds from common stock issuance | $					$ 802,000
LPC [Member] | Common Stock [Member] | Commitment Purchase Agreement [Member] | Maximum [Member] | Registration Rights Agreement [Member]
Equity [Line Items]
Total of purchase agreement and potential aggregate future purchases | $					$ 15,000,000
Common stock to be sold, subject to certain limitations					75,000
Total purchase proceeds per purchase date, subject to certain limitations | $					$ 1,000,000
LPC [Member] | Common Stock [Member] | Commitment Purchase Agreement [Member] | Minimum [Member] | Registration Rights Agreement [Member]
Equity [Line Items]
Percentage of shares may not sell if it is beneficial owning					4.99%
Tranche One [Member]
Equity [Line Items]
Number of common stock for issuance in exchange for future services from founders									500,000
Remaining number of repurchased shares								183,095
Stock compensation expense | $										$ 0	$ 0	$ 0	$ 0		$ 31,000
Tranche Two [Member]
Equity [Line Items]
Number of common stock for issuance in exchange for future services from founders									500,000
Remaining number of repurchased shares								183,095