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Pension and other post-retirement benefits - Weighted Average Assumptions Used to Determine Net Benefit Cost (Detail) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Discount rate | 3.19% | 4.19%
Expected return on assets | 6.85% | 6.87%
Rate of compensation increase | 3.96% | 4.00%
Health care cost trend rate |  |
Before Age 65 | 6.125% | 6.25%
Age 65 and after | 6.125% | 6.25%
Assumed Ultimate Medical Inflation Rate | 4.75% | 4.75%
Year in which Ultimate Rate is reached | 2031 | 2031
Other Postretirement Benefit Plans, Defined Benefit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Discount rate | 3.29% | 4.25%
Expected return on assets | 5.57% | 6.51%

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- Definition Defined Benefit Plan Health Care Cost Trend Rate Assumed For Next Fiscal Year Post Sixty Five + References No definition available. + Details
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- Definition Defined Benefit Plan Health Care Cost Trend Rate Assumed For Next Fiscal Year Pre Sixty Five + References No definition available. + Details
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- Definition Defined Benefit Plan, Ultimate Health Care Cost, Inflation 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 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 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 Weighted average rate of compensation increase used to determine net periodic benefit cost 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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