Employee Benefit Plans and Other Postretirement Benefits - Schedule of Pension Plan Asset Investments and Target Allocations (Details)	Dec. 31, 2025	Dec. 31, 2024
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Fair Value	93.00%	87.00%
Fixed Income Securities \| Packaging Corporation of America
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	94.00%
Domestic Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Fair Value	3.00%	7.00%
International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Fair Value	2.00%	3.00%
Other Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Fair Value	2.00%	3.00%
Other Securities \| Packaging Corporation of America
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	1.00%
Equity Securities \| Packaging Corporation of America
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%

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