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Retirement plans and postretirement benefits (Narrative) (Details) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) Y | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of increase in salaries based on the final average wages | 6.00% |  |
Percentage of equity in GTI | 50.00% |  |
Option to remain in defined benefit plan for an additional period, in years | 5 years |  |
Defined benefit plan, accumulated benefit obligation | $ 158,600 | $ 157,000 |
Employee retirement age, years | Y | 65 |  |
Basic contribution to savings plan by employees in percentage | 5.00% |  |
Additional basic contribution to savings plan by employees in percentage | 45.00% |  |
Pension and other postretirement benefit contributions | $ 1,600 | 2,500 |
Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investment plan percentage | 100.00% |  |
U.S. |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 109,845 | 100,905 | $ 93,897
U.S. | Equity Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investment plan percentage | 20.00% |  |
U.S. | Fixed Income Securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Investment plan percentage | 80.00% |  |
Foreign |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 13,618 | $ 11,871 | $ 11,293

X
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