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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, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
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% | 3.00%
Rate of compensation increase, year five and thereafter |  | 4.00% | 4.00%
Rate of compensation increase, year one through five |  |  | 3.00%
Rate of compensation increase, year six and thereafter |  |  | 4.00%
Defined Benefit Pension Plans [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Measurement date | 12/31/2014 | 12/31/2013 | 12/31/2012
Fiscal year end | 12/31/2015 | 12/31/2014 | 12/31/2013
Discount rate | 4.05% | 4.89% | 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 | 7.16% | 7.25% | 7.25%
Other Postretirement Benefit Plans [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Measurement date | 12/31/2014 | 12/31/2013 | 12/31/2012
Fiscal year end | 12/31/2015 | 12/31/2014 | 12/31/2013
Discount rate | 4.00% | 4.99% | 4.01%
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 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 Six 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 interest rate used to adjust for the time value of money. + 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=58740216&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512046 Reference 2: 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=58740216&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + 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=58740216&loc=d3e1928-114920 + Details
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