Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Erasca Foundation	IPO	Convertible Preferred Stock	Common Stock	Common Stock Erasca Foundation	Common Stock IPO	Additional Paid-in Capital	Additional Paid-in Capital Erasca Foundation	Additional Paid-in Capital IPO	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Convertible preferred stock, beginning balance, shares at Dec. 31, 2020				69,584,682
Convertible preferred stock, beginning balance, value at Dec. 31, 2020				$ 221,405
Beginning balance, shares at Dec. 31, 2020					25,189,673
Beginning balance at Dec. 31, 2020	$ (113,984)				$ 3			$ 1,413			$ 2	$ (115,402)
Issuance of Series B-1 and B-2 convertible preferred stock for cash, shares				15,931,772
Issuance of Series B-1 and B-2 convertible preferred stock for cash				$ 119,393
Issuance of common stock in connection with asset acquisition, shares					500,000
Issuance of common stock in connection with asset acquisition	1,680							1,680
Exercise of stock options, shares					439,610
Exercise of stock options	94							94
Vesting of early exercised stock options	174							174
Stock-based compensation expense	795							795
Net loss	(18,017)											(18,017)
Unrealized loss on marketable securities, net	(1)										(1)
Convertible preferred stock, ending balance, shares at Mar. 31, 2021				85,516,454
Convertible preferred stock,ending balance, value at Mar. 31, 2021				$ 340,798
Ending balance, shares at Mar. 31, 2021					26,129,283
Ending balance at Mar. 31, 2021	(129,259)				$ 3			4,156			1	(133,419)
Convertible preferred stock, beginning balance, shares at Dec. 31, 2020				69,584,682
Convertible preferred stock, beginning balance, value at Dec. 31, 2020				$ 221,405
Beginning balance, shares at Dec. 31, 2020					25,189,673
Beginning balance at Dec. 31, 2020	(113,984)				$ 3			1,413			2	(115,402)
Net loss	(92,300)
Ending balance, shares at Sep. 30, 2021					121,250,375
Ending balance at Sep. 30, 2021	482,991				$ 12			690,681				(207,702)
Convertible preferred stock, beginning balance, shares at Mar. 31, 2021				85,516,454
Convertible preferred stock, beginning balance, value at Mar. 31, 2021				$ 340,798
Beginning balance, shares at Mar. 31, 2021					26,129,283
Beginning balance at Mar. 31, 2021	(129,259)				$ 3			4,156			1	(133,419)
Issuance of common stock in connection with license agreement, shares					944,945
Issuance of common stock in connection with license agreement	5,488							5,488
Exercise of stock options, shares					225,895
Exercise of stock options	180							180
Vesting of early exercised stock options	174							174
Stock-based compensation expense	1,636							1,636
Net loss	(28,214)											(28,214)
Unrealized loss on marketable securities, net	(2)										(2)
Convertible preferred stock, ending balance, shares at Jun. 30, 2021				85,516,454
Convertible preferred stock,ending balance, value at Jun. 30, 2021				$ 340,798
Ending balance, shares at Jun. 30, 2021					27,300,123
Ending balance at Jun. 30, 2021	(149,997)				$ 3			11,634			(1)	(161,633)
Temporary equity stock issued during period shares conversion of convertible securities, shares				(85,516,454)
Temporary equity stock issued during period value conversion of convertible securities				$ (340,798)
Conversion of convertible preferred stock into common stock upon initial public offering, shares					71,263,685
Conversion of convertible preferred stock into common stock upon initial public offering	340,798				$ 7			340,791
Issuance of common stock, net of discounts and issuance costs, Shares						1,093,557	21,562,500
Issuance of common stock, net of discounts and issuance costs		$ 17,497	$ 316,999				$ 2		$ 17,497	$ 316,997
Disgorgement of stockholder's short-swing profits	553							553
Exercise of stock options, shares					30,510
Exercise of stock options	21							21
Vesting of early exercised stock options	171							171
Stock-based compensation expense	3,017							3,017
Net loss	(46,069)											(46,069)
Unrealized loss on marketable securities, net	1										1
Ending balance, shares at Sep. 30, 2021					121,250,375
Ending balance at Sep. 30, 2021	$ 482,991				$ 12			690,681				(207,702)
Convertible preferred stock, beginning balance, shares at Dec. 31, 2021				0
Beginning balance, shares at Dec. 31, 2021	121,382,547				121,382,547
Beginning balance at Dec. 31, 2021	$ 456,528				$ 12			694,844			(162)	(238,166)
Exercise of stock options, shares					357,244
Exercise of stock options	460							460
Vesting of early exercised stock options	479							479
Repurchases of early exercised stock options and restricted stock, Shares					(6,945)
Stock-based compensation expense	4,442							4,442
Net loss	(36,458)											(36,458)
Unrealized loss on marketable securities, net	(789)										(789)
Ending balance, shares at Mar. 31, 2022					121,732,846
Ending balance at Mar. 31, 2022	$ 424,662				$ 12			700,225			(951)	(274,624)
Convertible preferred stock, beginning balance, shares at Dec. 31, 2021				0
Beginning balance, shares at Dec. 31, 2021	121,382,547				121,382,547
Beginning balance at Dec. 31, 2021	$ 456,528				$ 12			694,844			(162)	(238,166)
Net loss	$ (107,555)
Convertible preferred stock, ending balance, shares at Sep. 30, 2022				0
Ending balance, shares at Sep. 30, 2022	122,306,969				122,306,969
Ending balance at Sep. 30, 2022	$ 365,319				$ 12			712,407			(1,379)	(345,721)
Beginning balance, shares at Mar. 31, 2022					121,732,846
Beginning balance at Mar. 31, 2022	424,662				$ 12			700,225			(951)	(274,624)
Exercise of stock options, shares					179,985
Exercise of stock options	375							375
Issuance of common stock under the Employee Stock Purchase Plan, Shares					150,642
Issuance of common stock under the Employee Stock Purchase Plan	617							617
Vesting of early exercised stock options	238							238
Stock-based compensation expense	5,055							5,055
Net loss	(35,608)											(35,608)
Unrealized loss on marketable securities, net	(272)										(272)
Ending balance, shares at Jun. 30, 2022					122,063,473
Ending balance at Jun. 30, 2022	395,067				$ 12			706,510			(1,223)	(310,232)
Exercise of stock options, shares					243,496
Exercise of stock options	427							427
Vesting of early exercised stock options	238							238
Stock-based compensation expense	5,232							5,232
Net loss	(35,489)											(35,489)
Unrealized loss on marketable securities, net	$ (156)										(156)
Convertible preferred stock, ending balance, shares at Sep. 30, 2022				0
Ending balance, shares at Sep. 30, 2022	122,306,969				122,306,969
Ending balance at Sep. 30, 2022	$ 365,319				$ 12			$ 712,407			$ (1,379)	$ (345,721)