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Employee benefit plans - Weighted-Average Asset Allocations (Details) | Dec. 31, 2023 | Dec. 31, 2022
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United States |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 100.00% | 100.00%
RTS Packaging Pension Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 100.00% |
U.K. |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 100.00% | 100.00%
Canada |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 100.00% | 100.00%
Retirement Plans | United States | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 22.20% | 23.40%
Retirement Plans | United States | Debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 76.60% | 72.90%
Retirement Plans | United States | Cash and short-term investments |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 1.20% | 3.70%
Retirement Plans | RTS Packaging Pension Plan | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 18.60% |
Retirement Plans | RTS Packaging Pension Plan | Debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 81.20% |
Retirement Plans | RTS Packaging Pension Plan | Cash and short-term investments |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 0.20% |
Retirement Plans | U.K. | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 20.00% | 22.60%
Retirement Plans | U.K. | Debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 79.10% | 76.30%
Retirement Plans | U.K. | Cash and short-term investments |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 0.90% | 1.10%
Retirement Plans | Canada | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 29.40% | 34.00%
Retirement Plans | Canada | Debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 70.60% | 66.00%
Retirement Plans | Canada | Cash and short-term investments |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 0.00% | 0.00%
Retiree Health and Life Insurance Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 100.00% | 100.00%
Retiree Health and Life Insurance Plans | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 0.00% | 0.00%
Retiree Health and Life Insurance Plans | Debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 100.00% | 100.00%
Retiree Health and Life Insurance Plans | Cash and short-term investments |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total (percent) | 0.00% | 0.00%

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- Definition 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. + References No definition available. + Details
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- Definition Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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