Consolidated Statements of Changes in Equity - USD ($) $ in Millions		Total	Total Shareholders' Equity	Common Stock	Capital in Excess of Par Value	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Loss	Noncontrolling Interest (“NCI”)
Beginning balance (in shares) at Dec. 31, 2021	[1]			259,000,000
Beginning balance at Dec. 31, 2021	[1]	$ 4,929	$ 4,914	$ 0	$ 3,485	$ (69)	$ 2,444	$ (946)	$ 15
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		1,035	1,034				1,034		1
Other comprehensive loss, net of tax		(263)	(263)					(263)
Share-based compensation		66	66		66
Issuance of common stock (in shares)				1,000,000
Shares distributed by Smurfit Kappa Employee Trust		0	0		(23)	23
Purchases of treasury stock		(32)	(32)			(32)
Share buyback		(42)	(42)			(42)
Cancellation of common stock (in shares)				(1,000,000)
Cancellation of common stock		0	0			42	(42)
Dividends		(350)	(349)				(349)		(1)
Ending balance (in shares) at Dec. 31, 2022				259,000,000
Ending balance at Dec. 31, 2022		5,343	5,328	$ 0	3,528	(78)	3,087	(1,209)	15
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		826	825				825		1
Other comprehensive loss, net of tax		362	362					362
Share-based compensation		64	64		64
Issuance of common stock (in shares)				1,000,000
Shares distributed by Smurfit Kappa Employee Trust		0	0		(17)	17
Purchases of treasury stock		(30)	(30)			(30)
Dividends		$ (391)	(391)				(391)
Ending balance (in shares) at Dec. 31, 2023		260,354,342		260,000,000
Ending balance at Dec. 31, 2023		$ 6,174	6,158	$ 0	3,575	(91)	3,521	(847)	16
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		319	319				319
Other comprehensive loss, net of tax		(808)	(808)					(808)
Share-based compensation		200	200		200
Shares distributed by Smurfit Kappa Employee Trust		0	0		(25)	25
Purchases of treasury stock		(27)	(27)			(27)
Shares of Smurfit Westrock common stock issued to WestRock shareholders and NCI assumed as a result of the Merger (in shares)				258,000,000
Shares of Smurfit Westrock common stock issued to WestRock shareholders and NCI assumed as a result of the Merger		12,110	12,099	$ 1	12,098				11
Converted WestRock RSUs and Options attributable to pre-Combination services		91	91		91
Issuance of common stock net of tax paid in connection with shares withheld from employees (in shares)				2,000,000
Issuance of common stock net of tax paid in connection with shares withheld from employees		(22)	(22)		4		(26)
Reclassification from retained earnings to accumulated other comprehensive loss		0	0				(209)	209
Dividends	[2]	$ (650)	(650)		5		(655)
Ending balance (in shares) at Dec. 31, 2024		520,444,261		520,000,000
Ending balance at Dec. 31, 2024		$ 17,387	$ 17,360	$ 1	$ 15,948	$ (93)	$ 2,950	$ (1,446)	$ 27

[1] Pursuant to the Transaction Agreement, on July 5, 2024 each issued ordinary share, par value €0.001 per share, of Smurfit Kappa (a “Smurfit Kappa Share”) was exchanged for one ordinary share, par value $0.001 per share, of Smurfit Westrock (a “Smurfit Westrock Share”). The exchange of shares is reflected retroactively to the earliest period presented.

[2] Includes cash dividends and dividend equivalent units declared on certain unvested share-based payment awards.