Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pensions
Pension and other postretirement benefits expenses
Service cost	$ 1,620	$ 1,356	$ 4,704	$ 4,064
Interest cost	3,815	4,539	11,438	13,647
Expected return on plan assets	(7,594)	(7,389)	(22,391)	(21,816)
Amortization of prior service cost	128	101	281	302
Amortization of actuarial losses	3,502	2,215	10,148	6,658
Curtailment loss	0	0	484	0
Settlement loss	0	93	0	340
Net periodic benefit cost	1,471	915	4,664	3,195
Other Postretirement Benefits
Pension and other postretirement benefits expenses
Service cost	20	18	61	53
Interest cost	259	335	780	1,008
Amortization of prior service cost	6	6	20	19
Amortization of actuarial losses	9	3	27	10
Net periodic benefit cost	$ 294	$ 362	$ 888	$ 1,090