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Income taxes - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2015
Dec. 31, 2017
Income Taxes [Line Items]        
U.S. corporate statutory income tax rate 21.00% 34.00%    
Effective income tax rate, percentage 0.00% 0.00%    
Percentage of tax benefits likelihood of being realized upon settlement of tax authority greater than 50%      
Unrecognized tax benefits $ 1,134,000     $ 1,134,000
Loss on disposal of discontinued operation, income taxes 0 $ 0    
Discontinued Operations, Disposed of [Member] | Advertising Business [Member]        
Income Taxes [Line Items]        
Tax loss resulting from the the disposal of discontinued operation     $ 4,375,000  
Loss on disposal of discontinued operation, income taxes     $ 0  
Accrued interest and penalties to uncertain tax positions $ 0