Pension and other postretirement benefits - Weighted average assumptions used in determining net periodic benefit cost (Detail)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.20%	4.30%	3.60%
Rate of return	7.50%	7.70%	7.80%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.10%	4.20%	3.60%
Initial healthcare trend rate	7.00%	7.40%	7.80%
Ultimate healthcare trend rate	5.00%	5.00%	5.00%

Defined benefit plan, assumptions used calculating net periodic benefit cost, initial healthcare trend rate.

No definition available.

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- Definition

Defined Benefit Plan, Net Periodic Benefit Cost, Ultimate Health Care Cost Trend Rate

+ References

No definition available.

+ Details

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- Definition

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

+ References

+ Details

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost 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)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

+ 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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