v3.8.0.1
Income taxes - Deferred tax assets and liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2018
Dec. 31, 2014
Deferred tax assets:            
Net operating loss carryforwards   $ 23,838 $ 23,669      
Outside basis differences for U.S. partnerships   14,306 16,121      
Stock options   4,100 5,307      
Deferred revenue   748 526      
Deferred compensation   249 199      
State taxes   78 49      
Other   1,282 1,622      
Valuation allowance   (34,990) (36,331) $ (19,548)   $ (12,470)
Net deferred tax assets   9,611 11,162      
Deferred tax liabilities:            
Intangible assets   (3,632) (5,658)      
Property and equipment   (6,983) (8,712)      
Net deferred tax liabilities   (10,615) (14,370)      
Net deferred taxes   $ (1,004) $ (3,208)      
Corporate federal tax rate   34.00% 34.00% 34.00%    
Income tax benefit resulting from reduction of corporate tax rate   $ 1,274        
Income tax benefit from indefinite carryforward of NOLs expected to recover deferred tax liabilities   $ (1,766)        
Forecast            
Deferred tax liabilities:            
Corporate federal tax rate 21.00%          
Taxable income which can be offset by operating losses carryforward (as a percent) 80.00%          
Executive compensation limit for covered employees $ 1,000          
Permitted bonus depreciation         100.00%  
Maximum interest expense deductible, as percent to taxable income 30.00%