Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 3,071,722	$ 2,519,906
Other comprehensive loss before reclassifications		77
Amounts reclassified from AOCL	1,078	(14,695)
Total other comprehensive income	1,078	(14,618)
Ending balance	3,151,265	2,618,849
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(126,065)	(113,090)
Other comprehensive loss before reclassifications		0
Amounts reclassified from AOCL	1,078	724
Total other comprehensive income	1,078	724
Ending balance	(124,987)	(112,366)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	532	455
Other comprehensive loss before reclassifications		77
Amounts reclassified from AOCL	0	0
Total other comprehensive income	0	77
Ending balance	532	532
Available-For-Sale Investment
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	15,419
Other comprehensive loss before reclassifications		0
Amounts reclassified from AOCL	0	(15,419)
Total other comprehensive income	0	(15,419)
Ending balance	0	0
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(125,533)	(97,216)
Total other comprehensive income	1,078	(14,618)
Ending balance	$ (124,455)	$ (111,834)