Accumulated other comprehensive loss - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 1,590,377	$ 1,340,924
Amounts reclassified from AOCL	1,121	1,069
Other comprehensive income, net of tax	1,422	1,079
Ending balance	1,658,256	1,405,251
Retirement Plans
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(142,398)	(136,893)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from AOCL	1,121	1,069
Other comprehensive income, net of tax	1,121	1,069
Ending balance	(141,277)	(135,824)
Foreign Currency Translation
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(199)	382
Other comprehensive income before reclassifications	301	10
Amounts reclassified from AOCL	0	0
Other comprehensive income, net of tax	301	10
Ending balance	102	392
Total
Increase (Decrease) in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(142,597)	(136,511)
Other comprehensive income before reclassifications	301	10
Amounts reclassified from AOCL	1,121	1,069
Other comprehensive income, net of tax	1,422	1,079
Ending balance	$ (141,175)	$ (135,432)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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