Other comprehensive income (loss) (reclassification adjustments) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Reclassification adjustments [Line Items]
Total reclassifications, net of tax	$ (330)	$ (139)
Prior service credit [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	355	355
Net actuarial loss [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(407)	(575)
Postretirement benefit plans [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(52)	(220)
Tax (expense) benefit	(31)	12
Total reclassifications, net of tax	(83)	(208)
Realized (loss) gain on interest rate swap [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(334)	93
Tax (expense) benefit	87	(24)
Total reclassifications, net of tax	$ (247)	$ 69

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