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Pension and postretirement benefits - Contributions to the benefit plans (Details) - USD ($) $ in Thousands | 9 Months Ended |
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Sep. 30, 2019 | Dec. 31, 2019
Pension Plans |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Employer contribution | $ 20,057 |
Contributions to the plans expected to pay |  | $ 20,229
OPEB Plan |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Employer contribution | $ 1,472 |
Contributions to the plans expected to pay |  | $ 8,128

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- Definition Amount of contributions expected to be received by defined benefit plan from employer in fiscal year. Excludes contributions paid by employer in current fiscal year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920 + Details
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