Quarterly report pursuant to Section 13 or 15(d)

Selected Balance Sheet Data - Additional Information (Detail)

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Selected Balance Sheet Data - Additional Information (Detail)
 3 Months Ended 6 Months Ended
Jan. 01, 2018
Jan. 01, 2017
Jan. 01, 2014
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Installments
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Mar. 31, 2013
USD ($)
Schedule Of Accrued Expenses [Line Items]                  
SARs frozen liability amount       $ 18,931,000   $ 18,931,000   $ 20,217,000  
Interest expense       352,000 $ 374,000 $ 712,000 $ 756,000    
Maximum payment deferral period for certain commissions payable           3 years      
SARs [Member]                  
Schedule Of Accrued Expenses [Line Items]                  
SARs frozen liability amount                 $ 20,000,000
SARs liability frozen value date           Mar. 31, 2013      
SARs liability interest accrual commencement date           Jan. 01, 2014      
Interest expense       $ 224,000 $ 233,000 $ 449,000 $ 466,000    
Treasury note term     10 years            
Base spread on SARs liability variable rate     2.00%            
SARs liability interest accrual rates 4.409% 4.446%              
SARs liability number of annual installments | Installments           10      
Estimated payouts description           Estimated payouts within the next twelve months for participants that have separated from service have been classified as current.      
Payments made during the period           $ 1,500,000      
Deferred Compensation Liability [Member]                  
Schedule Of Accrued Expenses [Line Items]                  
Estimated payouts description           Estimated payouts within the next twelve months for participants that have separated from service have been classified as current.      
Payments made during the period           $ 387,000      
Deferred Compensation Liability, Minimum Payout Period           2 years      
Deferred Compensation Liability, Maximum Payout Period           15 years      
Fair value of deferred compensation plan assets           110.00%