Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,741,723	$ 2,258,956	$ 2,519,906	$ 2,058,095
Other comprehensive income before reclassifications		40,254	77	40,812
Amounts reclassified from AOCL	766	1,657	(13,123)	3,571
Total other comprehensive income (loss)	766	41,911	(13,046)	44,383
Ending balance	2,878,255	2,419,707	2,878,255	2,419,707
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(111,560)	(119,065)	(113,090)	(120,979)
Other comprehensive income before reclassifications		0	0	0
Amounts reclassified from AOCL	766	1,657	2,296	3,571
Total other comprehensive income (loss)	766	1,657	2,296	3,571
Ending balance	(110,794)	(117,408)	(110,794)	(117,408)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	532	461	455	(97)
Other comprehensive income before reclassifications		(39)	77	519
Amounts reclassified from AOCL	0	0	0	0
Total other comprehensive income (loss)	0	(39)	77	519
Ending balance	532	422	532	422
Available-For-Sale Investment
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0	15,419	0
Other comprehensive income before reclassifications		40,293	0	40,293
Amounts reclassified from AOCL	0	0	(15,419)	0
Total other comprehensive income (loss)	0	40,293	(15,419)	40,293
Ending balance	0	40,293	0	40,293
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(111,028)	(118,604)	(97,216)	(121,076)
Total other comprehensive income (loss)	766	41,911	(13,046)	44,383
Ending balance	$ (110,262)	$ (76,693)	$ (110,262)	$ (76,693)