Pension Plans and Other Postretirement Benefits (Details-2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Components of Net Periodic Benefit Cost
Service cost	$ 5,383	$ 5,365	$ 7,263
Interest cost	12,625	12,393	13,766
Expected return on assets	(22,963)	(22,365)	(22,682)
Amortization of:
Transition obligation	0	0	0
Prior service (credit) cost	(17)	10	23
Actuarial loss	6,352	5,600	5,395
Net periodic benefit cost (credit)	1,380	1,003	3,765
Postretirement Benefits [Member]
Components of Net Periodic Benefit Cost
Service cost	1,231	1,271	1,559
Interest cost	2,666	2,681	3,075
Expected return on assets	(2,887)	(2,829)	(2,812)
Amortization of:
Transition obligation	3	3	3
Prior service (credit) cost	(2,669)	(2,669)	(2,669)
Actuarial loss	660	589	953
Net periodic benefit cost (credit)	$ (996)	$ (954)	$ 109