Stock Incentive Plans - Schedule of Stock Compensation Expense (Details) - Performance Shares - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 7	$ 3	$ 19	$ (1)
Location, Statement of Income, Balance [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	7	2	19	2
Location, Statement of Income, Balance [Axis]: gpk:BusinessCombinationsBusinessExitShutdownAndOtherSpecialChargesNet
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 0	$ 1	$ 0	$ (3)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details