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Notes payable - Additional Information (Details) - USD ($)
Apr. 30, 2020
Dec. 31, 2020
May 31, 2020
Dec. 31, 2019
Debt Disclosure [Abstract]        
Principal payments on notes in 2021   $ 1,900,000    
Principal payments on notes in 2022   900,000    
Principal payments on notes in 2023   100,000    
Principal payments on notes in 2025   98,900,000    
Debt Instrument [Line Items]        
Capital lease obligation   1,911,000   $ 3,716,000
Bank line of credit facility, outstanding   98,875,000   98,875,000
Notes payable, other than banks        
Debt Instrument [Line Items]        
Outstanding balances of short-term loan agreements   700,000   $ 8,000,000.0
Minimum        
Debt Instrument [Line Items]        
Commitment fee (percent) 0.20%      
Maximum        
Debt Instrument [Line Items]        
Commitment fee (percent) 0.35%      
LIBOR        
Debt Instrument [Line Items]        
Line of credit, conditional increase in maximum borrowing capacity     $ 50,000,000.0  
Line of credit        
Debt Instrument [Line Items]        
Bank line of credit facility amount $ 150,000,000.0   $ 200,000,000.0  
Fixed charge ratio, minimum 1.15      
Leverage ratio, maximum 3.50      
Aggregate annual capital expenditures allowed $ 30,000,000.0      
Annual dividend payments allowed $ 40,000,000.0      
Bank line of credit facility, outstanding   98,900,000    
Remaining borrowing capacity   98,600,000    
Line of credit | Letter of credit        
Debt Instrument [Line Items]        
Remaining borrowing capacity   $ 2,500,000    
Line of credit | Alternate Base Rate | Minimum        
Debt Instrument [Line Items]        
Basis spread on variable rate, line of credit (percent) 0.375%      
Line of credit | Alternate Base Rate | Maximum        
Debt Instrument [Line Items]        
Basis spread on variable rate, line of credit (percent) 0.50%      
Line of credit | LIBOR | Minimum        
Debt Instrument [Line Items]        
Basis spread on variable rate, line of credit (percent) 1.375%      
Line of credit | LIBOR | Maximum        
Debt Instrument [Line Items]        
Basis spread on variable rate, line of credit (percent) 1.75%