Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Retirement Benefits [Abstract]
Service cost-benefits earned during the period	$ 2	$ 0
Interest cost on benefit obligation	4,858	5,750
Expected return on plan assets	(7,750)	(6,575)
Amortization of prior service (credit) cost	(42)	25
Amortization of actuarial loss	1,600	1,500
(Gains from) expense for company-sponsored retirement plans	$ (1,332)	$ 700