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Pension and other postretirement benefits - Company matching contribution to defined contribution 401(K) plans (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Retirement Benefits [Abstract] |  |  |
Age requirement for future accruals | 50 years |  |
Company matching contribution to defined contribution (401(k)) plans | $ 4,505 | $ 3,945 | $ 5,446

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- Definition Defined Contribution Plan, Age Requirement for Future Accruals + References No definition available. + Details
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- References No definition available. + Details
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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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