Condensed Consolidated Statements of Redeemable Convertible Preferred Shares, Redeemable Noncontrolling Interest and Stockholders' Equity/ Members' (Deficit) (unaudited) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock [Member]	Redeemable Noncontrolling Interest [Member]	Common Stock [Member]	Common Stock [Member] Redeemable Convertible Preferred Stock [Member]	Common Stock [Member] Redeemable Noncontrolling Interest [Member]	Common Shares [Member]	Incentive Shares	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2020	$ (54,205)				$ 91,964	$ 5,702	$ 2,000	$ 637		$ (56,842)
Beginning Balance (In shares) at Dec. 31, 2020		22,851,257					6,035,869	4,112,012
Issuance of incentive shares								874,335
Share-based compensation expenses	538							$ 538
Issuance of Series B redeemable convertible preferred shares for cash, net of issuance costs		$ 129,757
Issuance of Series B redeemable convertible preferred shares for cash, net of issuance costs, shares		9,638,141
Net loss attributable to redeemable noncontrolling interests			$ (919)
Net loss attributable to Day One Biopharmaceuticals Holding Company, LLC members	(15,182)									(15,182)
Ending Balance at Mar. 31, 2021	(68,849)				221,721	4,783	$ 2,000	$ 1,175		(72,024)
Ending Balance (In shares) at Mar. 31, 2021		32,489,398					6,035,869	4,986,352
Beginning Balance at Dec. 31, 2020	(54,205)				91,964	5,702	$ 2,000	$ 637		(56,842)
Beginning Balance (In shares) at Dec. 31, 2020		22,851,257					6,035,869	4,112,012
Unrealized loss on available-for-sale securities	0
Net loss attributable to redeemable noncontrolling interests	(2,109)
Net loss attributable to common stockholders/members	(148,701)
Ending Balance at Sep. 30, 2021	297,672			$ 6					$ 403,214	(105,548)
Ending Balance (In shares) at Sep. 30, 2021				61,928,939
Beginning Balance at Mar. 31, 2021	(68,849)				$ 221,721	$ 4,783	$ 2,000	$ 1,175		(72,024)
Beginning Balance (In shares) at Mar. 31, 2021		32,489,398					6,035,869	4,986,352
Issuance of incentive shares								2,085,460
Cancellations of incentive shares								(265,596)
Share-based compensation expenses	2,537								2,537
Conversion of redeemable convertible preferred, common, and incentive shares into common stock , Value	221,721	$ (221,721)		$ 4			$ (2,000)	$ 1,175	224,892
Conversion of redeemable convertible preferred, common, and incentive shares into common stock (In shares)		(32,489,398)		43,958,557			(6,035,869)	6,806,216
Conversion of redeemable noncontrolling interest to common stock	3,592		(3,592)	$ 1					3,592
Conversion of redeemable noncontrolling interest to common stock (In shares)				6,470,382
Net loss attributable to Day One Biopharmaceuticals Holding Company, LLC members			$ (1,191)
Common stock issued in IPO, net of issuance costs of $16,995	167,045			$ 1					167,044
Common stock issued in IPO, net of issuance costs of $16,995 (In shares)				11,500,000
Net loss attributable to common stockholders/members	(14,284)									(14,284)
Ending Balance at Jun. 30, 2021	311,763			$ 6					398,065	(86,308)
Ending Balance (In shares) at Jun. 30, 2021				61,928,939
Share-based compensation expenses	5,149								5,149
Unrealized loss on available-for-sale securities	0
Net loss attributable to redeemable noncontrolling interests	0
Net loss attributable to common stockholders/members	(19,240)									(19,240)
Ending Balance at Sep. 30, 2021	297,672			$ 6					403,214	(105,548)
Ending Balance (In shares) at Sep. 30, 2021				61,928,939
Beginning Balance at Dec. 31, 2021	281,148						$ 6		408,629	(127,487)
Beginning Balance (In shares) at Dec. 31, 2021							61,952,292
Share-based compensation expenses	6,202								6,202
Unvested stock forfeiture							(40,363)
Net loss attributable to common stockholders/members	(27,747)									(27,747)
Ending Balance at Mar. 31, 2022	259,603						$ 6		414,831	(155,234)
Ending Balance (In shares) at Mar. 31, 2022							61,911,929
Beginning Balance at Dec. 31, 2021	281,148						$ 6		408,629	(127,487)
Beginning Balance (In shares) at Dec. 31, 2021							61,952,292
Unrealized loss on available-for-sale securities	(392)
Net loss attributable to redeemable noncontrolling interests	0
Net loss attributable to common stockholders/members	(102,072)
Ending Balance at Sep. 30, 2022	363,076						$ 7		593,020	(229,559)	$ (392)
Ending Balance (In shares) at Sep. 30, 2022							73,511,923
Beginning Balance at Mar. 31, 2022	259,603						$ 6		414,831	(155,234)
Beginning Balance (In shares) at Mar. 31, 2022							61,911,929
Share-based compensation expenses	5,631								5,631
Unrealized loss on available-for-sale securities	(3)										(3)
Issuance of common stock pursuant to follow-on offering, net of issuance costs, Shares							11,500,000
Issuance of common stock pursuant to follow-on offering, net of issuance costs, Amount	161,610						$ 1		161,609
Issuance of common stock pursuant to Employee Stock Purchase Plan, Shares							49,171
Issuance of common stock pursuant to Employee Stock Purchase Plan, Amount	320								320
Net loss attributable to common stockholders/members	(36,530)									(36,530)
Ending Balance at Jun. 30, 2022	390,631						$ 7		582,391	(191,764)	(3)
Ending Balance (In shares) at Jun. 30, 2022							73,461,100
Share-based compensation expenses	8,576								8,576
Unrealized loss on available-for-sale securities	(389)										(389)
Issuance of Series B redeemable convertible preferred shares for cash, net of issuance costs, shares							130,899
Net loss attributable to redeemable noncontrolling interests	0
Issuance of common stock pursuant to follow-on offering, net of issuance costs, Amount									2,053
Conversion of redeemable convertible preferred, common, and incentive shares into common stock (In shares)							51,030
Common stock issued in IPO, net of issuance costs of $16,995	2,053
Unvested stock forfeiture							(131,106)
Net loss attributable to common stockholders/members	(37,795)									(37,795)
Ending Balance at Sep. 30, 2022	$ 363,076						$ 7		$ 593,020	$ (229,559)	$ (392)
Ending Balance (In shares) at Sep. 30, 2022							73,511,923