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Pension Plans - Projected Benefit Obligation and Fair Value of Plan Assets (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Funded Status Of Plan [Abstract]      
Projected benefit obligation at beginning of the year $ 34,858 $ 34,763  
Service cost 255 203 $ 189
Interest cost 1,456 1,465 1,436
Actuarial (gain) loss 2,215 (516)  
Benefits paid (2,753) (1,057)  
Projected benefit obligation at end of year 36,031 34,858 34,763
Fair value of plan assets at beginning of the year 24,435 23,136  
Actual return on plan assets 3,460 1,365  
Employer contributions 225 991  
Benefits paid (2,753) (1,057)  
Fair value of plan assets at end of the year 25,367 24,435 $ 23,136
Unfunded pension obligation $ 10,664 $ 10,423