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Pension and other postretirement benefits - Employer contributions (Details) - USD ($) | 12 Months Ended | 45 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |  |  |
Pension contributions | $ 0 | $ 34,595,000 | $ 0 |  |
PBGC Agreement [Member] |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |  |  |
Required pension contributions above minimum |  |  |  |  | $ 17,400,000
Pension contributions | 0 | $ 1,100,000 | $ 0 | $ 6,700,000 |
Pension contributions above minimum remaining | $ 9,600,000 |  |  |  |

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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 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 The amount of cash or cash equivalents contributed by the entity to fund its pension plans. + 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=66902209&loc=d3e3536-108585 + Details
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