Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	PRNB
Entity Registrant Name	Principia Biopharma Inc.
Entity Central Index Key	0001510487
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		33,040,758
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Title of 12(b) Security	Common Stock, Par Value $0.0001 Per Share
Security Exchange Name	NASDAQ
Entity File Number	001-38653
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-3487603
Entity Address, Address Line One	220 East Grand Avenue
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	416-7700
Document Quarterly Report	true
Document Transition Report	false