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Pension and other post-retirement benefits - Weighted Average Assumptions Used to Determine Net Benefit Obligation (Detail) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Discount rate | 4.19% | 3.43%
Interest crediting rate (for cash balance plans) | 4.43% | 4.50%
Rate of compensation increase | 4.00% | 3.00%
Rate of compensation increase | 3.00% | 3.00%
Other Postretirement Benefit Plans, Defined Benefit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Discount rate | 4.26% | 3.60%
Health care cost trend rate |  |
Before age 65 | 6.25% | 6.25%
Age 65 and after | 6.25% | 6.25%
Assumed ultimate medical inflation rate | 4.75% | 4.75%
Year in which ultimate rate is reached | 2031 | 2024

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- Definition Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Interest Crediting Rate + References No definition available. + Details
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- Definition Defined Benefit Plan, Benefit Obligation Ultimate Health Care Cost Inflation Rate + References No definition available. + Details
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- Definition Defined Benefit Plan, Benefit Obligation, Year that Rate Reaches Ultimate Trend Rate + References No definition available. + Details
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- Definition Defined Benefit Plan Health Care Cost Trend Rate Assumed For Calculating Benefit Obligation in 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 Calculating Benefit Obligation in Next Fiscal Year Pre Sixty Five + 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 benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + 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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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