Summary of significant accounting policies - Company Held Significant Investment (Details)	Dec. 31, 2020
RTS Packaging JVCO
Noncontrolling Interest [Line Items]
Ownership interest (percent)	35.00%
Cascades Conversion, Inc.
Noncontrolling Interest [Line Items]
Ownership interest (percent)	50.00%
Cascades Sonoco, Inc.
Noncontrolling Interest [Line Items]
Ownership interest (percent)	50.00%
Showa Products Company Ltd.
Noncontrolling Interest [Line Items]
Ownership interest (percent)	22.20%
Papertech Energía, S.L.
Noncontrolling Interest [Line Items]
Ownership interest (percent)	25.00%
Weidenhammer New Packaging, LLC
Noncontrolling Interest [Line Items]
Ownership interest (percent)	40.00%