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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
	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

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No definition available.

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- References

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- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

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No definition available.

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