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Employee Benefits - Additional Information (Details)
 12 Months Ended
Dec. 19, 2017
USD ($)
retiree
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Defined Benefit Plan Disclosure [Line Items]        
Number of consecutive years of employee's compensation   5 years    
Number of total years of employee's compensation   10 years    
Net actuarial loss included in accumulated other comprehensive losses expected to be recognized   $ 2,200,000    
Future prior service credit to be recognized due to freeze of accrual benefits   0    
Accumulated benefit obligation   106,700,000 $ 105,800,000  
Number of retirees covered by annuity contract | retiree 124      
Number of retirees covered by annuity contract receiving death benefits | retiree 30      
Defined benefit obligations, death benefits $ 2,000      
Total premium paid out of plan's assets $ 1,500,000 8,244,000 5,250,000  
Pension contributions   0    
Projected benefit obligation   106,664,000 105,834,000 $ 109,747,000
Net periodic benefit cost   (347,000) 296,000 1,736,000
Transfers of plan assets between levels 1 and 2 on recurring basis   0 0  
Purchases, transfers or sales of level 3 plan assets   0    
Supplemental Executive Retirement Plan        
Defined Benefit Plan Disclosure [Line Items]        
Projected benefit obligation   5,300,000 4,700,000  
Net periodic benefit cost   500,000 500,000 600,000
Amounts before tax reflected in accumulated other comprehensive income (loss)   $ 2,700,000 2,500,000 2,100,000
Thrift Plan        
Defined Benefit Plan Disclosure [Line Items]        
Contributions to the Thrift Plan   3.50%    
Compensation expense   $ 1,800,000 $ 1,700,000 $ 1,500,000
Marketable equity securities | Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Targeted asset allocation percentage   50.00%    
Marketable equity securities | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Targeted asset allocation percentage   70.00%    
Fixed Income | Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Targeted asset allocation percentage   30.00%    
Fixed Income | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Targeted asset allocation percentage   50.00%