Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,139,338	$ 1,658,256	$ 2,058,095	$ 1,590,377
Other comprehensive income before reclassifications	189	(205)	558	96
Amounts reclassified from AOCL	1,005	1,201	1,914	2,322
Total other comprehensive income	1,194	996	2,472	2,418
Ending balance	2,258,956	1,675,307	2,258,956	1,675,307
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(120,070)	(141,277)	(120,979)	(142,398)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,005	1,201	1,914	2,322
Total other comprehensive income	1,005	1,201	1,914	2,322
Ending balance	(119,065)	(140,076)	(119,065)	(140,076)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	272	102	(97)	(199)
Other comprehensive income before reclassifications	189	(205)	558	96
Amounts reclassified from AOCL	0	0	0	0
Total other comprehensive income	189	(205)	558	96
Ending balance	461	(103)	461	(103)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(119,798)	(141,175)	(121,076)	(142,597)
Total other comprehensive income	1,194	996	2,472	2,418
Ending balance	$ (118,604)	$ (140,179)	$ (118,604)	$ (140,179)