Organization and Description of Business - Additional Information (Detail) $ / shares in Units, $ in Thousands			9 Months Ended		12 Months Ended
	Jun. 05, 2020 USD ($) $ / shares shares	May 22, 2020	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reverse stock split, description			1-for-7.15
Reverse stock split, conversion ratio		0.13986014	0.1399
Common stock, shares issued			35,465,715		1,846,024
Proceeds from issuance of common stock | $				$ 146
Initial Public Offering
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reverse stock split, conversion ratio	7.15
Initial public offering, shares issued	10,350,000
Share price of shares issued in initial public offering | $ / shares	$ 16.00
Proceeds from the initial public offering | $	$ 148,300
Underwriting discounts, commissions and offering expenses | $	$ 5,700		$ 3,100		$ 2,600
Convertible preferred stock converted into common shares	160,389,279
Reclassification of convertible preferred stock into additional paid in capital upon initial public offering | $	$ 238,300
Initial Public Offering | Underwriters
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Initial public offering, shares issued	1,350,000
Initial Public Offering | Common Stock
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Initial public offering, shares issued	9,000,000
Private Placement | Novartis
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Common stock, shares issued	625,000
Share price | $ / shares	$ 16.00
Proceeds from issuance of common stock | $	$ 10,000
Private Placement | Pliant Therapeutics, Inc. | Novartis
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of noncontrolling ownership	6.10%