Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 3,151,265	$ 2,618,849	$ 3,071,722	$ 2,519,906
Other comprehensive loss before reclassifications			2,066	77
Amounts reclassified from AOCL	988	806	2,066	(13,889)
Other comprehensive income (loss), net of tax	988	806	2,066	(13,812)
Ending balance	2,904,158	2,741,723	2,904,158	2,741,723
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(124,455)	(111,834)	(125,533)	(97,216)
Other comprehensive income (loss), net of tax	988	806	2,066	(13,812)
Ending balance	(123,467)	(111,028)	(123,467)	(111,028)
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(124,987)	(112,366)	(126,065)	(113,090)
Other comprehensive loss before reclassifications			2,066	0
Amounts reclassified from AOCL	988	806		1,530
Other comprehensive income (loss), net of tax	988	806	2,066	1,530
Ending balance	(123,999)	(111,560)	(123,999)	(111,560)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	532	532	532	455
Other comprehensive loss before reclassifications			0	77
Amounts reclassified from AOCL	0	0		0
Other comprehensive income (loss), net of tax	0	0	0	77
Ending balance	532	532	532	532
Available-For-Sale Investment
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0	0	15,419
Other comprehensive loss before reclassifications			0	0
Amounts reclassified from AOCL	0	0		(15,419)
Other comprehensive income (loss), net of tax	0	0	0	(15,419)
Ending balance	$ 0	$ 0	$ 0	$ 0