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] 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]
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 A redeemable convertible preferred shares, shares		9,638,141
Net loss attributable to redeemable convertible noncontrolling interests	(919)		$ (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, 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
Net loss attributable to redeemable convertible noncontrolling interests	0
Unvested stock forfeiture						(40,363)
Net loss attributable to common stockholders	(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