Pension and Other Postretirement Benefits (Details 11) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Pension and other postretirement benefits expenses
Service cost	$ 6,055	$ 5,395	$ 5,508
Interest cost	18,819	19,494	20,019
Expected return on plan assets	(28,082)	(30,302)	(32,404)
Amortization of prior service cost	446	210	305
Recognized losses	10,557	10,791	15,004
Curtailment loss gain	(7,217)	0	0
Settlement loss	(119)	0	9,939
Net periodic benefit cost	459	5,588	18,371
Estimated net actuarial loss for the defined benefit pension plans that will be amortized from accumulated other non-owner changes to equity into net periodic benefit cost in 2017	11,222
Estimated prior service cost for the defined benefit pension plans that will be amortized from accumulated other non-owner changes to equity into net periodic benefit cost (credit) in 2017	562
Other postretirement benefit plans [Member]
Pension and other postretirement benefits expenses
Service cost	83	122	145
Interest cost	1,561	1,766	1,836
Expected return on plan assets	0	0	0
Amortization of prior service cost	(68)	(373)	(564)
Recognized losses	276	535	1,011
Curtailment loss gain	0	0	0
Settlement loss	0	0	0
Net periodic benefit cost	1,852	$ 2,050	$ 2,428
Estimated net actuarial loss for the defined benefit pension plans that will be amortized from accumulated other non-owner changes to equity into net periodic benefit cost in 2017	641
Estimated prior service cost for the defined benefit pension plans that will be amortized from accumulated other non-owner changes to equity into net periodic benefit cost (credit) in 2017	$ 20