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Fair Value Measurements (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Financial Liabilities [Abstract]    
Net unrealized gains (losses) incurred $ 149 $ 242
Amount of investments, Level 1 to Level 2 transfer amount 0  
Amount of investments, Level 2 to Level 1 transfer amount 0  
Measured at Fair Value on a Recurring Basis [Member]    
Financial Assets [Abstract]    
Derivative financial instruments 538  
Total financial assets 17,534 16,177
Financial Liabilities [Abstract]    
SERP liabilities 8,913 8,552
Derivative financial instruments   112
Total financial liabilities 8,913 8,664
Measured at Fair Value on a Recurring Basis [Member] | Corporate Bonds [Member]    
Financial Assets [Abstract]    
Trading debt securities 5,495 5,661
Measured at Fair Value on a Recurring Basis [Member] | Municipal Bonds [Member]    
Financial Assets [Abstract]    
Trading debt securities 1,522 1,912
Measured at Fair Value on a Recurring Basis [Member] | Floating Rate Notes [Member]    
Financial Assets [Abstract]    
Trading debt securities 1,556 753
Measured at Fair Value on a Recurring Basis [Member] | Asset Backed Securities [Member]    
Financial Assets [Abstract]    
Trading debt securities 478 526
Measured at Fair Value on a Recurring Basis [Member] | U.S. Treasury Notes [Member]    
Financial Assets [Abstract]    
Trading debt securities 1,306 1,030
Measured at Fair Value on a Recurring Basis [Member] | Other [Member]    
Financial Assets [Abstract]    
Trading debt securities 852 968
Measured at Fair Value on a Recurring Basis [Member] | Preferred Stocks [Member]    
Financial Assets [Abstract]    
Trading equity securities 283 364
Measured at Fair Value on a Recurring Basis [Member] | Supplemental Employee Retirement Plan [Member] | Money Market Fund [Member]    
Financial Assets [Abstract]    
Trading equity securities 257 124
Measured at Fair Value on a Recurring Basis [Member] | Supplemental Employee Retirement Plan [Member] | Mutual Funds [Member]    
Financial Assets [Abstract]    
Trading equity securities 5,247 4,839
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member]    
Financial Assets [Abstract]    
Derivative financial instruments 0  
Total financial assets 14,144 12,771
Financial Liabilities [Abstract]    
SERP liabilities 0 0
Derivative financial instruments   0
Total financial liabilities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Corporate Bonds [Member]    
Financial Assets [Abstract]    
Trading debt securities 5,495 5,661
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Municipal Bonds [Member]    
Financial Assets [Abstract]    
Trading debt securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Floating Rate Notes [Member]    
Financial Assets [Abstract]    
Trading debt securities 1,556 753
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Asset Backed Securities [Member]    
Financial Assets [Abstract]    
Trading debt securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | U.S. Treasury Notes [Member]    
Financial Assets [Abstract]    
Trading debt securities 1,306 1,030
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Other [Member]    
Financial Assets [Abstract]    
Trading debt securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Preferred Stocks [Member]    
Financial Assets [Abstract]    
Trading equity securities 283 364
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Supplemental Employee Retirement Plan [Member] | Money Market Fund [Member]    
Financial Assets [Abstract]    
Trading equity securities 257 124
Measured at Fair Value on a Recurring Basis [Member] | Level 1 [Member] | Supplemental Employee Retirement Plan [Member] | Mutual Funds [Member]    
Financial Assets [Abstract]    
Trading equity securities 5,247 4,839
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member]    
Financial Assets [Abstract]    
Derivative financial instruments 538  
Total financial assets 3,390 3,406
Financial Liabilities [Abstract]    
SERP liabilities 8,913 8,552
Derivative financial instruments   112
Total financial liabilities 8,913 8,664
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Corporate Bonds [Member]    
Financial Assets [Abstract]    
Trading debt securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Municipal Bonds [Member]    
Financial Assets [Abstract]    
Trading debt securities 1,522 1,912
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Floating Rate Notes [Member]    
Financial Assets [Abstract]    
Trading debt securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Asset Backed Securities [Member]    
Financial Assets [Abstract]    
Trading debt securities 478 526
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | U.S. Treasury Notes [Member]    
Financial Assets [Abstract]    
Trading debt securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Other [Member]    
Financial Assets [Abstract]    
Trading debt securities 852 968
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Preferred Stocks [Member]    
Financial Assets [Abstract]    
Trading equity securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Supplemental Employee Retirement Plan [Member] | Money Market Fund [Member]    
Financial Assets [Abstract]    
Trading equity securities 0 0
Measured at Fair Value on a Recurring Basis [Member] | Level 2 [Member] | Supplemental Employee Retirement Plan [Member] | Mutual Funds [Member]    
Financial Assets [Abstract]    
Trading equity securities $ 0 $ 0