Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 1,658,256	$ 1,405,251	$ 1,590,377	$ 1,340,924
Amounts reclassified from AOCL	1,201	1,591	2,322	2,660
Other comprehensive income, net of tax	996	1,238	2,418	2,317
Ending balance	1,675,307	1,479,742	1,675,307	1,479,742
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(141,277)	(135,824)	(142,398)	(136,893)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,201	1,591	2,322	2,660
Other comprehensive income, net of tax	1,201	1,591	2,322	2,660
Ending balance	(140,076)	(134,233)	(140,076)	(134,233)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	102	392	(199)	382
Other comprehensive loss before reclassifications	(205)	(353)	96	(343)
Amounts reclassified from AOCL	0	0	0	0
Other comprehensive income, net of tax	(205)	(353)	96	(343)
Ending balance	(103)	39	(103)	39
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(141,175)	(135,432)	(142,597)	(136,511)
Other comprehensive loss before reclassifications	(205)	(353)	96	(343)
Amounts reclassified from AOCL	1,201	1,591	2,322	2,660
Other comprehensive income, net of tax	996	1,238	2,418	2,317
Ending balance	$ (140,179)	$ (134,194)	$ (140,179)	$ (134,194)