Notes payable - Additional Information (Details)	1 Months Ended
	Nov. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
Principal payments on notes in 2019		$ 5,400,000
Principal payments on notes in 2020		1,800,000
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
Debt Instrument [Line Items]
Capital lease obligations		9,161,000		$ 10,437,000
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		2,600,000		4,300,000
Minimum
Debt Instrument [Line Items]
Commitment fee (as a percent)	0.20%
Maximum
Debt Instrument [Line Items]
Commitment fee (as a percent)	0.35%
Capital Lease Obligations
Debt Instrument [Line Items]
Capital lease obligations		6,500,000		$ 6,100,000
Line of Credit
Debt Instrument [Line Items]
Bank line of credit facility amount	$ 150,000,000		$ 125,000,000
Line of credit, conditional increase in maximum borrowing capacity	$ 50,000,000
Fixed charge ratio, minimum	1.15
Leverage ratio, maximum	3.5
Aggregate annual capital expenditures allowed	$ 30,000,000
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