Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Retirement Benefits [Abstract]
Service cost-benefits earned during the period	$ 1	$ 2	$ 1	$ 4
Interest cost on benefit obligation	3,988	4,879	7,938	9,737
Expected return on plan assets	(8,690)	(7,779)	(17,340)	(15,529)
Amortization of prior service cost	20	87	45	45
Amortization of actuarial loss	1,246	1,481	2,446	3,081
Income from company-sponsored retirement plans	$ (3,435)	$ (1,330)	$ (6,910)	$ (2,662)