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Stock Options and Other Equity-Based Incentive Instruments - Additional Information (Detail) - USD ($)
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Options currently not exercisable yet expected to vest 1,125  
Restricted Stock Units (RSUs) [Member] | Market Plus Service Condition Vesting RSUs [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of units outstanding expected to vest 45,536 45,536
Weighted-average remaining contractual term 1 year 2 months 12 days  
Pre-tax compensation costs $ 633,000  
Restricted Stock Units (RSUs) [Member] | Service Condition Vesting RSUs [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of units outstanding expected to vest 66,914 66,947
Options available for grant under the plan 66,914  
Weighted-average remaining contractual term 1 year 2 months 12 days  
Pre-tax compensation costs $ 1,465,000  
Number of units released 355  
Maximum [Member] | Restricted Stock Units (RSUs) [Member] | Market Plus Service Condition Vesting RSUs [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of units released 68,304