Employee benefit plans - Weighted average assumptions to determine benefit obligations (Details)	Dec. 31, 2020	Dec. 31, 2019
Pension
Employee benefit plans
Discount rate	2.20%	3.00%
Other Postretirement
Employee benefit plans
Discount rate	2.20%	3.00%

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant