Consolidated Statements of Redeemable Convertible Preferred Shares, Redeemable Noncontrolling Interest and Stockholders Equity/ Members (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock [Member]	Redeemable Noncontrolling Interest [Member]	Common Stock	Common Stock Redeemable Convertible Preferred Stock [Member]	Common Stock Redeemable Noncontrolling Interest [Member]	Common Shares [Member]	Incentive Shares [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2018	$ (150)			$ 0	$ 0	$ 0		$ 0	$ 0	$ (150)
Beginning Balance (In shares) at Dec. 31, 2018		0		0			9,323,724	32,026
Issuance of Series A redeemable convertible preferred shares for cash net of issuance costs of $95 and derivative tranches liability of $1,483		$ 28,422
Issuance of Series A redeemable convertible preferred shares, shares		10,348,507
Issuance of Series A redeemable convertible preferred shares upon notes conversion		$ 2,082
Issuance of Series A redeemable convertible preferred shares upon notes conversion, shares		2,154,245
Recognition of contingent beneficial conversion feature (Note [ ])	2,000						$ 2,000
Proceeds from redeemable convertible preferred stock			$ 9,837
Issuance of incentive shares, shares								1,456,395
Stock repurchased during period, shares							(3,287,855)
Issuance of shares pursuant to Employee Stock Purchase Plan, Shares								1,456,395
Share-based compensation expense	111							$ 111
Net loss attributable to redeemable convertible noncontrolling interests	(4,350)		(4,350)
Net loss attributable to Day One Biopharmaceuticals Holding Company, LLC members	(12,634)									(12,634)
Ending Balance at Dec. 31, 2019	(10,673)				30,504	5,487	$ 2,000	$ 111		(12,784)
Ending Balance (In shares) at Dec. 31, 2019		12,502,752		0			6,035,869	1,488,421
Issuance of Series A redeemable convertible preferred shares for cash net of issuance costs of $95 and derivative tranches liability of $1,483		$ 29,977
Issuance of Series A redeemable convertible preferred shares, shares		10,348,505
Recognition of contingent beneficial conversion feature (Note [ ])	0
Issuance of incentive shares, shares								3,101,178
Cancellations of incentive shares, shares								(477,582)
Reclassification of derivative tranches liability upon settlement		$ 31,483
Issuance of shares pursuant to Employee Stock Purchase Plan, Shares								3,101,178
Share-based compensation expense	526							$ 526
Net loss attributable to redeemable convertible noncontrolling interests	(3,336)		(3,336)
Transfer to redeemable noncontrolling interest related to change in ownership	(3,551)		(3,551)							(3,551)
Net loss attributable to Day One Biopharmaceuticals Holding Company, LLC members	(40,507)									(40,507)
Temporary Equity, Ending Balance, Shares at Dec. 31, 2020		22,851,257
Ending Balance at Dec. 31, 2020	(54,205)			$ 0	91,964	5,702	$ 2,000	$ 637	0	(56,842)
Ending Balance (In shares) at Dec. 31, 2020		22,851,257		0			6,035,869	4,112,017
Issuance of Series A redeemable convertible preferred shares for cash net of issuance costs of $95 and derivative tranches liability of $1,483		$ 129,757
Issuance of Series A redeemable convertible preferred shares, shares		9,638,141
Recognition of contingent beneficial conversion feature (Note [ ])	0
Issuance of incentive shares	318								318
Issuance of incentive shares, shares				23,353				2,959,795
Cancellations of incentive shares, shares								(265,596)
Conversion of redeemable convertible preferred, common, and incentive shares into common stock	221,721	$ (221,721)		$ 4			$ (2,000)	$ (1,175)	224,892
Conversion of redeemable convertible preferred, common, and incentive shares into common stock, Shares		(32,489,398)		43,958,557			(6,035,869)	(6,806,216)
Conversion of redeemable noncontrolling interest to common stock	3,593		(3,593)	$ 1					3,592
Conversion Of Redeemable Noncontrolling Interest To Common Stock Shares				6,470,382
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, shares				11,500,000
Issuance of shares pursuant to Employee Stock Purchase Plan	318								318
Issuance of shares pursuant to Employee Stock Purchase Plan, Shares				23,353				2,959,795
Net loss attributable to common stockholders/members	(70,645)									(70,645)
Share-based compensation expense	13,321							$ 538	12,783
Net loss attributable to redeemable convertible noncontrolling interests	(2,109)		$ (2,109)
Temporary Equity, Ending Balance, Shares at Dec. 31, 2021		0
Ending Balance at Dec. 31, 2021	$ 281,148			$ 6	$ 0	$ 0	$ 0	$ 0	$ 408,629	$ (127,487)
Ending Balance (In shares) at Dec. 31, 2021		0		61,952,292			0	0