Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pensions
Pension and other postretirement benefits expenses
Service cost	$ 1,538	$ 1,435	$ 3,279	$ 3,084
Interest cost	3,204	3,806	6,376	7,623
Expected return on plan assets	(6,962)	(7,404)	(13,934)	(14,797)
Amortization of prior service cost	80	73	165	153
Amortization of actuarial losses	4,080	3,307	8,006	6,646
Curtailment loss	0	484	0	484
Net periodic benefit cost	1,940	1,701	3,892	3,193
Other Postretirement Benefits
Pension and other postretirement benefits expenses
Service cost	25	19	50	41
Interest cost	206	257	412	521
Amortization of prior service cost	8	7	15	14
Amortization of actuarial losses	70	(5)	140	18
Net periodic benefit cost	$ 309	$ 278	$ 617	$ 594