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Stockholders' equity - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2010
Accumulated Other Comprehensive Income Loss [Line Items]      
Number of shares authorized for repurchase     2,000,000
Shares available for repurchase under the plan 1,840,697 1,951,000  
Aggregate purchase price $ 1.4    
Common stock      
Accumulated Other Comprehensive Income Loss [Line Items]      
Stock repurchased and acquired in market transactions 110,303    
Aggregate purchase price $ 1.4