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Pension and other post-retirement benefits - Weighted Average Assumptions Used to Determine Net Benefit Obligation (Detail)	12 Months Ended
	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

X

- Definition

Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Interest Crediting Rate

+ References

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- Definition

Defined Benefit Plan, Benefit Obligation Ultimate Health Care Cost Inflation Rate

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- Definition

Defined Benefit Plan, Benefit Obligation, Year that Rate Reaches Ultimate Trend Rate

+ References

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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

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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

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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

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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

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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

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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.

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