Reclassification out of accumulated other comprehensive loss (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	$ (25,260)	$ (23,453)
Income taxes	(7,950)	(12,949)
Other income	(2,400)	3,354
Net income	31,763	52,131
Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Net income	(589)	(612)
Reclassification adjustment for loss on derivative instruments included in net income | Reclassification out of accumulated other comprehensive loss | Interest rate contract
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	(148)	(148)
Income taxes	36	37
Net income	(112)	(111)
Amortization of postretirement liability losses included in net periodic benefit credit | Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Income taxes	164	151
Other income	(609)	(617)
Net income	(445)	(466)
Reclassification adjustment on available-for-sale investments included in net income | Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Income taxes	8	9
Other income	(40)	(44)
Net income	$ (32)	$ (35)