Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pensions [Member]
Pension and other postretirement benefits expenses
Service cost	$ 1,275	$ 1,398	$ 2,708	$ 2,988
Interest cost	4,572	4,383	9,108	8,692
Expected return on plan assets	(7,349)	(7,383)	(14,427)	(14,777)
Amortization of prior service cost (credit)	98	140	201	281
Amortization of actuarial (gains) losses	2,285	3,029	4,443	5,840
Settlement loss	247	0	247	0
Net periodic benefit cost	1,128	1,567	2,280	3,024
Expected company contributions in 2018	20,000		20,000
Pensions [Member] | Qualified Plan [Member]
Pension and other postretirement benefits expenses
Expected company contributions in 2018	15,000		15,000
Other Postretirement Benefits [Member]
Pension and other postretirement benefits expenses
Service cost	16	18	35	42
Interest cost	333	337	673	681
Amortization of prior service cost (credit)	7	5	13	10
Amortization of actuarial (gains) losses	(3)	126	7	286
Net periodic benefit cost	$ 353	$ 486	$ 728	$ 1,019