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 convertible 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 convertible noncontrolling interests	(2,109)
Ending Balance at Jun. 30, 2021	311,763			$ 6					$ 398,065	(86,308)
Ending Balance (In shares) at Jun. 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
Unrealized loss on available-for-sale securities	0
Net loss attributable to redeemable convertible noncontrolling interests	(1,191)
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
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	(3)
Net loss attributable to redeemable convertible noncontrolling interests	0
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
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)
Net loss attributable to redeemable convertible noncontrolling interests	0
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 shares pursuant to Employee Stock Purchase Plan, Shares							49,171
Issuance of shares 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