Notes payable - Additional Information (Details) - USD ($)	1 Months Ended
	Nov. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2018
Debt Disclosure [Abstract]
Principal payments on notes in 2020		$ 9,800,000
Principal payments on notes in 2021		900,000
Principal payments on notes in 2022		900,000
Principal payments on notes in 2023		99,000,000.0
Debt Instrument [Line Items]
Capital lease obligation		3,716,000
Capital lease obligations			$ 6,500,000
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		8,000,000.0	$ 2,600,000
Minimum
Debt Instrument [Line Items]
Commitment fee (as a percent)	0.20%
Maximum
Debt Instrument [Line Items]
Commitment fee (as a percent)	0.35%
Line of Credit
Debt Instrument [Line Items]
Bank line of credit facility amount	$ 150,000,000.0			$ 125,000,000.0
Line of credit, conditional increase in maximum borrowing capacity	$ 50,000,000.0
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		48,600,000
Line of Credit | Base Rate | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate, line of credit (percent)	0.375%
Line of Credit | 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%
Line of Credit | Letter of Credit
Debt Instrument [Line Items]
Remaining borrowing capacity		$ 2,500,000