Reclassification out of accumulated other comprehensive loss (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Loss [Line Items]
Interest expense	$ (93,984)	$ (96,519)	$ (98,587)
Income taxes	(88,920)	(84,590)	(63,279)
Other income	26,416	26,711	15,812
Net income	378,131	390,205	$ 335,453
Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Loss [Line Items]
Net income	(2,450)	(2,420)
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	(591)	(591)
Income taxes	145	145
Net income	(446)	(446)
Amortization of postretirement liability losses included in net periodic benefit cost | Reclassification out of accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Loss [Line Items]
Income taxes	615	630
Other income	(2,485)	(2,552)
Net income	(1,870)	(1,922)
Reclassification adjustment for loss 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	36	14
Other income	(170)	(66)
Net income	$ (134)	$ (52)