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Employee benefit plans - Weighted-Average Asset Allocations (Details)	Dec. 31, 2023	Dec. 31, 2022
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.

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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

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