Employee benefit plans - Recognized Amounts in Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Total Recognized Amounts in the Consolidated Balance Sheets
Noncurrent liabilities	$ (304,798)	$ (374,419)
Retirement Plan
Total Recognized Amounts in the Consolidated Balance Sheets
Noncurrent assets	24,196	18,520
Current liabilities	(13,913)	(12,935)
Noncurrent liabilities	(302,960)	(371,030)
Net (liability)/asset	(292,677)	(365,445)
Retiree Health and Life Insurance Plans
Total Recognized Amounts in the Consolidated Balance Sheets
Noncurrent assets	0	0
Current liabilities	(784)	(983)
Noncurrent liabilities	(830)	(2,146)
Net (liability)/asset	$ (1,614)	$ (3,129)

Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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- Definition

Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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- Definition

Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

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