Income Taxes - Summary of Statutory Tax Rates Applicable to Income of Company and Subsidiaries (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Line Items]
Effective income tax rate	21.00%	21.00%
89Bio Ltd
Income Tax Disclosure [Line Items]
Effective income tax rate	23.00%	23.00%
89bio Management, Inc.
Income Tax Disclosure [Line Items]
Effective income tax rate	21.00%	21.00%
UAB 89bio Lithuania
Income Tax Disclosure [Line Items]
Effective income tax rate	15.00%	15.00%