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Retirement Plans - Schedule of Changes in Benefit Obligations and Plan Assets (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Service cost $ 0 $ 0 $ 0
Interest cost 49 67 62
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Funded Status at End of Year 1,194 1,394  
Unrecognized transition obligation 0 0  
Unrecognized net gain 440 345  
Amounts recognized in retained earnings 1,634 1,739  
Change in Benefit Obligation [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 1,394 1,815  
Service cost 0 0  
Interest cost 49 67  
Amendments 0 0  
Actuarial gain (116) (337)  
Net benefits paid (133) (151)  
Benefit obligation at end of year 1,194 1,394 1,815
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Net benefits paid (133) (151)  
Change In Plan Assets [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Net benefits paid (133) (151)  
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 0 0  
Actual return on plan assets 0 0  
Employer contributions 133 151  
Net benefits paid (133) (151)  
Fair value of plan assets at end of year $ 0 $ 0 $ 0