Reclassification out of accumulated other comprehensive loss (Details 2) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Interest expense	$ (28,113)		$ (23,381)		$ (53,373)	$ (46,835)
Income taxes	(17,579)		(23,164)		(25,528)	(36,112)
Other income	(4,246)		9,027		(6,646)	12,381
Net income	70,667	$ 31,763	100,190	$ 52,131	102,430	152,321
Reclassification out of accumulated other comprehensive loss
Reclassification adjustment out of accumulated other comprehensive loss [Line Items]
Net income	(597)		(594)		(1,186)	(1,206)
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	(147)		(148)		(295)	(296)
Income taxes	36		36		72	73
Net income	(111)		(112)		(223)	(223)
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	148		160		312	311
Other income	(609)		(617)		(1,218)	(1,234)
Net income	(461)		(457)		(906)	(923)
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	7		6		15	15
Other income	(32)		(31)		(72)	(75)
Net income	$ (25)		$ (25)		$ (57)	$ (60)