Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Service cost-benefits earned during the period	$ 1	$ 2	$ 5	$ 6
Interest cost on benefit obligation	4,868	5,761	14,605	17,284
Expected return on plan assets	(7,765)	(6,580)	(23,294)	(19,740)
Amortization of prior service cost	23	23	68	68
Amortization of actuarial loss	1,541	1,521	4,622	4,562
Pension payment timing related charge	0	0	0	686
(Gains from) expense for company-sponsored retirement plans	$ (1,332)	$ 727	$ (3,994)	$ 2,866