Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pensions [Member]
Pension and other postretirement benefits expenses
Service cost	$ 1,398	$ 1,459	$ 2,988	$ 3,102
Interest cost	4,383	4,666	8,692	9,287
Expected return on plan assets	(7,383)	(6,816)	(14,777)	(13,676)
Amortization of prior service cost (credit)	140	111	281	220
Amortization of actuarial losses	3,029	2,663	5,840	5,143
Curtailment gain	0	(7,217)	0	(7,217)
Settlement gain	0	(230)	0	(491)
Net periodic benefit cost	1,567	(5,364)	3,024	(3,632)
Expected company contributions in 2018	4,600		4,600
Pensions [Member] | Qualified Plan [Member]
Pension and other postretirement benefits expenses
Expected company contributions in 2018	0		0
Other Postretirement Benefits [Member]
Pension and other postretirement benefits expenses
Service cost	18	20	42	42
Interest cost	337	392	681	781
Amortization of prior service cost (credit)	5	(17)	10	(34)
Amortization of actuarial losses	126	70	286	139
Net periodic benefit cost	$ 486	$ 465	$ 1,019	$ 928