Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pensions
Pension and other postretirement benefits expenses
Service cost	$ 1,435	$ 1,275	$ 3,084	$ 2,708
Interest cost	3,806	4,572	7,623	9,108
Expected return on plan assets	(7,404)	(7,349)	(14,797)	(14,427)
Amortization of prior service cost	73	98	153	201
Amortization of actuarial losses	3,307	2,285	6,646	4,443
Curtailment loss	484	0	484	0
Settlement loss	0	247	0	247
Net periodic benefit cost	1,701	1,128	3,193	2,280
Other Postretirement Benefits
Pension and other postretirement benefits expenses
Service cost	19	16	41	35
Interest cost	257	333	521	673
Amortization of prior service cost	7	7	14	13
Amortization of actuarial losses	(5)	(3)	18	7
Net periodic benefit cost	$ 278	$ 353	$ 594	$ 728