Pension Plans and Other Postretirement Benefits (Details-3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Weighted-average assumptions used to determine benefit obligations:
Discount rate	3.73%	4.29%
Rate of compensation increase	3.67%	3.71%
Weighted-average assumptions used to determine net periodic cost:
Discount rate	4.30%	4.51%	4.11%
Long-term rate of return	7.40%	7.65%	7.80%
Rate of compensation increase	3.76%	3.76%	3.84%
Postretirement Benefits [Member]
Weighted-average assumptions used to determine benefit obligations:
Discount rate	3.58%	4.11%
Assumed health care cost trend rates:
Health care cost trend rate assumed for next year	6.00%	6.25%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	5.00%	5.00%
Year that the rate reaches the ultimate trend rate	2022	2022
Weighted-average assumptions used to determine net periodic cost:
Discount rate	4.09%	4.32%	3.96%
Long-term rate of return	6.80%	6.96%	7.06%
How assumed 1% increase or decrease in health care cost trends could impact postretirement benefits [Abstract]
Effect on other postretirement benefit obligation - 1% increase	$ 740
Effect on other postretirement benefit obligation - 1% decrease	(1,076)
Effect on total service and interest cost components - 1% increase	19
Effect on total service and interest cost components - 1% decrease	$ (31)