Pension Plans and Other Postretirement Benefits (Details-5) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 410,121	$ 355,110
Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	488	284
Equity Securities [Member] | U.S. Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	124,594	107,406
Equity Securities [Member] | U.S. Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	29,138	25,966
Equity Securities [Member] | U.S. Small Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	37,782	33,836
Equity Securities [Member] | International Blend [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	71,514	59,054
Fixed Income Securities [Member] | Short-Term Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	4,641	4,318
Fixed Income Securities [Member] | High Yield Bond [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	19,400	17,978
Fixed Income Securities [Member] | Long Duration Bond [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	91,678	71,512
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	25,995	29,441
Insurance Continuance Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	1,500	1,514
Fixed Rate Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
NAV of plan assets	$ 3,391	$ 3,801