Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 1,675,307	$ 1,479,742	$ 1,590,377	$ 1,340,924
Amounts reclassified from AOCL	1,161	1,073	3,483	3,733
Other comprehensive income, net of tax	1,092	835	3,510	3,152
Ending balance	1,799,536	1,521,696	1,799,536	1,521,696
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(140,076)	(134,233)	(142,398)	(136,893)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,161	1,073	3,483	3,733
Other comprehensive income, net of tax	1,161	1,073	3,483	3,733
Ending balance	(138,915)	(133,160)	(138,915)	(133,160)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(103)	39	(199)	382
Other comprehensive loss before reclassifications	(69)	(238)	27	(581)
Amounts reclassified from AOCL	0	0	0	0
Other comprehensive income, net of tax	(69)	(238)	27	(581)
Ending balance	(172)	(199)	(172)	(199)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(140,179)	(134,194)	(142,597)	(136,511)
Other comprehensive loss before reclassifications	(69)	(238)	27	(581)
Amounts reclassified from AOCL	1,161	1,073	3,483	3,733
Other comprehensive income, net of tax	1,092	835	3,510	3,152
Ending balance	$ (139,087)	$ (133,359)	$ (139,087)	$ (133,359)