Consolidated Statements of Redeemable Convertible Preferred Shares, Redeemable Noncontrolling Interest and Stockholders Equity/ Members (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Preferred Stock [Member] Redeemable Convertible Preferred Stock [Member]	Common Shares [Member]	Incentive Shares [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]	Redeemable Noncontrolling Interest [Member]
Beginning Balance at Dec. 31, 2019	$ (10,673)		$ 30,504	$ 2,000	$ 111		$ (12,784)		$ 5,487
Beginning Balance (In shares) at Dec. 31, 2019			12,502,752	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
Issuance of incentive shares, shares					3,101,178
Cancellations of incentive shares, shares					(477,582)
Reclassification of derivative tranches liability upon settlement			$ 31,483
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)
Unrealized loss on available-for-sale securities	0
Ending Balance at Dec. 31, 2020	(54,205)		$ 91,964	$ 2,000	$ 637		(56,842)		5,702
Ending Balance (In shares) at Dec. 31, 2020			22,851,257	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
Issuance of incentive shares, shares					2,959,795
Cancellations of incentive shares, shares					(265,596)
Conversion of redeemable convertible preferred, common, and incentive shares into common stock	221,721	$ 4	$ (221,721)	$ (2,000)	$ (1,175)	$ 224,892
Conversion of redeemable convertible preferred, common, and incentive shares into common stock, Shares		43,958,557	(32,489,398)	(6,035,869)	(6,806,216)
Conversion of redeemable noncontrolling interest to common stock	3,593	$ 1				3,592			(3,593)
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
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)
Stock Issued During Period, Shares, Conversion of Units		43,958,557	(32,489,398)	(6,035,869)	(6,806,216)
Unrealized loss on available-for-sale securities	0
Ending Balance at Dec. 31, 2021	281,148	$ 6				408,629	(127,487)
Ending Balance (In shares) at Dec. 31, 2021		61,952,292
Issuance of shares pursuant to Employee Stock Purchase Plan	642					642
Issuance of shares pursuant to Employee Stock Purchase Plan, Shares				97,413
Net loss attributable to common stockholders/members	(142,181)						(142,181)
Share-based compensation expense	27,242					27,242
Net loss attributable to redeemable convertible noncontrolling interests	0
Issuance of common stock upon exercise of stock options, shares		235,474
Issuance of common stock upon exercise of stock options, Amount	3,649					3,649
Issuance of common stock upon release of restricted stock units, shares		79,441
Common stock issued pursuant to follow-on offering, net of issuance costs, Shares				11,500,000
Common stock issued pursuant to follow-on offering, net of issuance costs, Amount	161,610			$ 1		161,609
Unvested common stock forfeiture, shares				406,444
Unrealized loss on available-for-sale securities	(71)							$ (71)
Ending Balance at Dec. 31, 2022	$ 332,039	$ 7				$ 601,771	$ (269,668)	$ 71
Ending Balance (In shares) at Dec. 31, 2022		73,458,176