Restructuring and Other Charges - Summary of Components of Restructuring Costs that are Recorded within Statement of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	$ (2,741)	$ 6,615
Employee-related [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	2,644	4,528
Asset-related [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	(5,385)	2,087
Research and development [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	(3,615)	6,213
Research and development [Member] \| Employee-related [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	1,770	4,126
Research and development [Member] \| Asset-related [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	(5,385)	2,087
General and administrative [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	874	402
General and administrative [Member] \| Employee-related [Member]
Restructuring Cost And Reserve [Line Items]
Total Restructuring cost incurred	$ 874	$ 402

FIVE PRIME THERAPEUTICS, INC. 10-K