Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 05, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q1
Entity Registrant Name	PHATHOM PHARMACEUTICALS, INC.
Entity Central Index Key	0001783183
Entity Tax Identification Number	82-4151574
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity File Number	001-39094
Entity Shell Company	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	true
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	100 Campus Drive
Entity Address, Address Line Two	Suite 102
Entity Address, City or Town	Florham Park
Entity Address, State or Province	NJ
Entity Address, Postal Zip Code	07932
City Area Code	877
Local Phone Number	742-8466
Entity Common Stock, Shares Outstanding		28,964,506
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	PHAT
Security Exchange Name	NASDAQ