Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,519,906	$ 2,058,095
Other comprehensive income before reclassifications	77	369
Amounts reclassified from AOCL	(14,695)	909
Other comprehensive income, net of tax	(14,618)	1,278
Ending balance	2,618,849	2,139,338
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(113,090)	(120,979)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from AOCL	724	909
Other comprehensive income, net of tax	724	909
Ending balance	(112,366)	(120,070)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	455	(97)
Other comprehensive income before reclassifications	77	369
Amounts reclassified from AOCL	0	0
Other comprehensive income, net of tax	77	369
Ending balance	532	272
Available-For-Sale Investment
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	15,419	0
Other comprehensive income before reclassifications	0	0
Amounts reclassified from AOCL	(15,419)	0
Other comprehensive income, net of tax	(15,419)	0
Ending balance	0	0
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(97,216)	(121,076)
Other comprehensive income, net of tax	(14,618)	1,278
Ending balance	$ (111,834)	$ (119,798)