Notes payable and line of credit - Additional Information (Details)		1 Months Ended	12 Months Ended
	Feb. 10, 2016 USD ($)	Feb. 28, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)
Debt Disclosure [Abstract]
Principal payments on notes in 2018			$ 7,400,000
Principal payments on notes in 2019			101,000,000
Principal payments on notes in 2020			900,000
Debt Instrument [Line Items]
Capital lease obligations			10,437,000		$ 13,933,000
Notes Payable, Other Than Banks
Debt Instrument [Line Items]
Outstanding balances of short-term loan agreements			4,300,000		6,500,000
Capital Lease Obligations
Debt Instrument [Line Items]
Capital lease obligations			6,100,000		$ 7,300,000
Line of Credit
Debt Instrument [Line Items]
Bank line of credit facility amount			125,000,000
Bank line of credit facility, outstanding			98,900,000
Remaining borrowing capacity			23,600,000
Line of credit, conditional increase in maximum borrowing capacity			$ 50,000,000
Fixed charge ratio, minimum		1.25
Leverage ratio, maximum		2.25
Aggregate annual capital expenditures allowed		$ 25,000,000
Maximum exception to allowed restricted payments for the new share repurchase program	$ 50,000,000
Annual dividend payments allowed		$ 35,000,000		$ 25,000,000
Line of Credit | Base Rate | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate, line of credit (percent)			0.50%
Line of Credit | Base Rate | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate, line of credit (percent)			1.00%
Line of Credit | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate, line of credit (percent)			1.50%
Line of Credit | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate, line of credit (percent)			2.00%
Line of Credit | Letter of Credit
Debt Instrument [Line Items]
Remaining borrowing capacity			$ 2,500,000