Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Retirement Benefits [Abstract]
Service cost-benefits earned during the period	$ 2	$ 0	$ 4	$ 0
Interest cost on benefit obligation	4,879	5,773	9,737	11,523
Expected return on plan assets	(7,779)	(6,585)	(15,529)	(13,160)
Amortization of prior service cost	87	20	45	45
Amortization of actuarial loss	1,481	1,541	3,081	3,041
Pension payment timing related charge	0	686	0	686
(Gains from) expense for company-sponsored retirement plans	$ (1,330)	$ 1,435	$ (2,662)	$ 2,135