Pension Plans and Other Postretirement Benefits (Details-5) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 481,273	$ 434,922
Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,180	964
Equity Securities [Member] | U.S. Large Cap [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151,218	134,458
Equity Securities [Member] | U.S. Mid Cap [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37,389	31,288
Equity Securities [Member] | U.S. Small Cap [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,456	36,495
Equity Securities [Member] | International Blend [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87,461	76,293
Fixed Income Securities [Member] | Short-Term Fund [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,215	7,449
Fixed Income Securities [Member] | High Yield Bond [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22,683	21,276
Fixed Income Securities [Member] | Long Duration Bond [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	92,657	92,642
Real Estate [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,005	29,671
Insurance Continuance Fund [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,506	1,530
Fixed Rate Fund [Member] | Fair Value Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,503	$ 2,856