Pension and other postretirement benefits - Weighted average assumptions used in determining benefit obligations (Detail)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Pension
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.50%	3.20%
Rate of compensation increase	2.80%	2.80%
Other Benefits
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.30%	3.10%
Initial healthcare trend rate	6.40%	6.60%
Ultimate healthcare trend rate	4.50%	5.00%

Defined benefit plan, assumptions used calculating benefit obligation, initial healthcare trend rate.

No definition available.

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- Definition

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

+ References

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

+ Details

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- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

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)(2)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ References

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

+ Details

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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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- Details

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- Details

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