Other comprehensive income (loss) (reclassification adjustments) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification adjustments [Line Items]
Total reclassifications, net of tax	$ (326)	$ (416)	$ (656)	$ (555)
Prior service credit [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	355	355	711	711
Net actuarial loss [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(407)	(575)	(814)	(1,150)
Postretirement benefit plans [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(52)	(220)	(103)	(439)
Tax (expense) benefit	(31)	12	(63)	23
Total reclassifications, net of tax	(83)	(208)	(166)	(416)
Realized loss gain on interest rate swap [Member]
Reclassification adjustments [Line Items]
Amounts reclassified from accumulated other comprehensive loss	(330)	(281)	(664)	(188)
Tax (expense) benefit	87	73	174	49
Total reclassifications, net of tax	$ (243)	$ (208)	$ (490)	$ (139)

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