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Pension and other postretirement benefits - Weighted average assumptions used to determine net periodic benefit cost (Details) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Rate of compensation increase, year one through two | 3.00% |  |
Rate of compensation increase, year three and thereafter |  | 4.00% |
Rate of compensation increase, year one through three |  | 3.00% |
Rate of compensation increase, year four and thereafter |  | 4.00% |
Rate of compensation increase, year one through four |  |  | 3.00%
Rate of compensation increase, year five and thereafter |  |  | 4.00%
Defined Benefit Pension Plans [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Measurement date | 12/31/2016 | 12/31/2015 | 12/31/2014
Fiscal year end | 12/31/2017 | 12/31/2016 | 12/31/2015
Discount rate | 4.15% | 4.44% | 4.05%
Rate of compensation increase, group 1 | 3.00% | 3.00% | 3.00%
Rate of compensation increase, group 2 | 4.00% | 4.00% | 4.00%
Expected return on plan assets | 6.82% | 7.10% | 7.16%
Other Postretirement Benefit Plans [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Measurement date | 12/31/2016 | 12/31/2015 | 12/31/2014
Fiscal year end | 12/31/2017 | 12/31/2016 | 12/31/2015
Discount rate | 4.05% | 4.50% | 4.00%
Rate of compensation increase, group 1 | 3.00% | 3.00% | 3.00%
Rate of compensation increase, group 2 | 4.00% | 4.00% | 4.00%
Expected return on plan assets | 0.00% | 0.00% | 0.00%

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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Five and Thereafter + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Four and Thereafter + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One Through Four + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One Through Three + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One Through Two + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Three and Thereafter + References No definition available. + Details
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- Definition The date(s) used to measure (at least the majority of) plan assets and benefit obligations for defined benefit pension and other postretirement benefit plans. + References No definition available. + Details
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- Definition The maximum expected rate of compensation increases (for pay-related plans). + References No definition available. + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost 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 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost 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)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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 + 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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