Other comprehensive (loss) income (reclassification adjustments) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Reclassification adjustments [Line Items]
Total reclassifications, net of tax	$ (139)	$ (381)
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	(575)	(806)
Postretirement benefit plans [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(220)	(451)
Tax benefit	12	70
Total reclassifications, net of tax	(208)	(381)
Realized gain on interest rate swap [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	93	0
Tax benefit	(24)	0
Total reclassifications, net of tax	$ 69	$ 0

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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