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Summary of Stock Option (Detail) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Number of shares, Beginning Balance 5,511  
Options granted, Shares 110  
Options forfeited, Shares (160)  
Options expired, Shares (123)  
Options exercised, Shares (55)  
Number of shares, Ending Balance 5,283 5,511
Weighted average exercise price, Beginning Balance $ 4.98  
Options granted, Weighted average exercise price 4.03  
Options forfeited, Weighted average exercise price 3.23  
Options expired, Weighted average exercise price 5.90  
Options exercised, Weighted average exercise price 3.30  
Weighted average exercise price, Ending Balance $ 5.01 $ 4.98
Weighted average remaining contractual term, End of the period 5 years 18 days 5 years 6 months 10 days