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Commitments and contingencies - Legal Contingencies (Details) - USD ($) |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Aug. 18, 2017 | Feb. 09, 2017 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Gain recognized |  |  |  | $ 5,500,000 | $ (3,830,000) | $ (30,850,000) |  |
Accrued and other current liabilities |  |  |  | 61,398,000 | 50,100,000 |  |  |
Pension contributions |  |  |  | 0 | 0 | 34,595,000 |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Litigation settlement amount | $ 23,000,000 |  |  |  |  |  |  |
Litigation settlement period | 10 years |  |  |  |  |  |  |
Ravenswood retiree legal settlement |  |  | $ 5,000,000 |  |  |  |  |
Gain recognized |  |  |  | 5,500,000 |  |  |  |
Loss contingency accrual |  |  |  | 12,500,000 |  |  |  |
Accrued and other current liabilities |  |  |  | $ 12,856,000 |  |  |  |
Litigation settlement, annual installment |  | $ 2,000,000 |  |  |  |  |  |
Litigation settlement installment period |  |  |  | 9 years |  |  |  |
PBGC Agreement [Member] |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Required pension contributions above minimum |  |  |  |  |  |  |  | $ 17,400,000
Pension contributions |  |  |  | $ 0 | $ 0 | $ 1,100,000 | $ 0 | $ 6,700,000
Pension contributions above minimum remaining |  |  |  | $ 9,600,000 |  |  |  |

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- Definition Litigation Settlement, Annual Installment + References No definition available. + Details
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- Definition Litigation Settlement Installment Period + References No definition available. + Details
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- Definition Litigation Settlement Payment Period + References No definition available. + Details
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- Definition Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, this amount represents the remaining additional contributions (above any minimum required contributions) to our defined benefit pension plans. + References No definition available. + Details
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- Definition Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, we will make additional contributions (above any minimum required contributions) to our defined benefit pension plans. + References No definition available. + Details
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- Definition Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 + Details
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- Definition Amount awarded to other party in judgment or settlement of litigation. + References No definition available. + Details
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- Definition 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. + References No definition available. + Details
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- Definition Amount of loss contingency liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 + Details
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- Definition The amount of cash paid for the settlement of litigation or for other legal issues during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
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- Definition Amount of payment for pension benefits. Excludes other postretirement benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
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