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Savings Plan (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 1998
Dec. 31, 1997
Savings Plan [Line Items]          
Charges recognized $ 3,500,000 $ 3,200,000 $ 3,000,000    
Number of common stock purchased       7,200 46,671
Amount of common stock purchased       $ 200,000 $ 1,000,000
Savings Plan          
Savings Plan [Line Items]          
Employee contribution to savings plan 1        
Defined Contribution Plan Employer Employee Contribution $ 1        
Maximum matching contribution 5.00%        
Percentage of enrollment of base pay 3.00%        
Nonqualified Plan [Member]          
Savings Plan [Line Items]          
Liability under the restoration plan $ 1,300,000 $ 1,600,000      
Liability under the restoration plan, shares 65,548 67,013