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Pension and other post-retirement benefits - Benefit Obligations in Excess of Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligation | $ 439,458 | $ 462,943
Fair value of plan assets | 339,099 | 376,276
Projected benefit obligation | 476,791 | 523,743
Fair value of plan assets | 339,099 | 403,945
Other Postretirement Benefit Plans, Defined Benefit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligation | 163,375 | 171,175
Fair value of plan assets | 107,430 | 121,561
Projected benefit obligation | 163,375 | 171,175
Fair value of plan assets | $ 107,430 | $ 121,561

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- Definition Amount of accumulated benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Fair value of plan assets for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Amount of benefit obligation for defined benefit plan with benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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