Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,618,849	$ 2,139,338	$ 2,519,906	$ 2,058,095
Other comprehensive income before reclassifications		189	77	558
Amounts reclassified from AOCL	806	1,005	(13,889)	1,914
Total other comprehensive income (loss)	806	1,194	(13,812)	2,472
Ending balance	2,741,723	2,258,956	2,741,723	2,258,956
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(112,366)	(120,070)	(113,090)	(120,979)
Other comprehensive income before reclassifications		0	0	0
Amounts reclassified from AOCL	806	1,005	1,530	1,914
Total other comprehensive income (loss)	806	1,005	1,530	1,914
Ending balance	(111,560)	(119,065)	(111,560)	(119,065)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	532	272	455	(97)
Other comprehensive income before reclassifications		189	77	558
Amounts reclassified from AOCL	0	0	0	0
Total other comprehensive income (loss)	0	189	77	558
Ending balance	532	461	532	461
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		0	0	0
Amounts reclassified from AOCL	0	0	(15,419)	0
Total other comprehensive income (loss)	0	0	(15,419)	0
Ending balance	0	0	0	0
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(111,834)	(119,798)	(97,216)	(121,076)
Total other comprehensive income (loss)	806	1,194	(13,812)	2,472
Ending balance	$ (111,028)	$ (118,604)	$ (111,028)	$ (118,604)