Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,258,956	$ 1,675,307	$ 2,058,095	$ 1,590,377
Other comprehensive income before reclassifications	40,254	(69)	40,812	27
Amounts reclassified from AOCL	1,657	1,161	3,571	3,483
Other comprehensive income, net of tax	41,911	1,092	44,383	3,510
Ending balance	2,419,707	1,799,536	2,419,707	1,799,536
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(119,065)	(140,076)	(120,979)	(142,398)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,657	1,161	3,571	3,483
Other comprehensive income, net of tax	1,657	1,161	3,571	3,483
Ending balance	(117,408)	(138,915)	(117,408)	(138,915)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	461	(103)	(97)	(199)
Other comprehensive income before reclassifications	(39)	(69)	519	27
Amounts reclassified from AOCL	0	0	0	0
Other comprehensive income, net of tax	(39)	(69)	519	27
Ending balance	422	(172)	422	(172)
Available-For-Sale Investment
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0	0	0
Other comprehensive income before reclassifications	40,293	0	40,293	0
Amounts reclassified from AOCL	0	0	0	0
Other comprehensive income, net of tax	40,293	0	40,293	0
Ending balance	40,293	0	40,293	0
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(118,604)	(140,179)	(121,076)	(142,597)
Other comprehensive income, net of tax	41,911	1,092	44,383	3,510
Ending balance	$ (76,693)	$ (139,087)	$ (76,693)	$ (139,087)