Notes payable and line of credit - Additional Information (Detail)		12 Months Ended
	Feb. 10, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Disclosure [Abstract]
Principal payments on notes in 2017		$ 9,700,000
Principal payments on notes in 2018		2,300,000
Principal payments on notes in 2019		94,100,000
Principal payments on notes in 2020		700,000
Debt Instrument [Line Items]
Capital lease obligations		13,933,000	$ 4,399,000
Additional cash issued to class B common stock holders		12,000,000	0	$ 0
Impairment of goodwill	$ 35,900,000	0	35,749,000	$ 0
Notes Payable, Other Than Banks
Debt Instrument [Line Items]
Outstanding balances of short-term loan agreements		6,500,000	400,000
Conversion, Class B Common Stock to Common Stock
Debt Instrument [Line Items]
Additional cash issued to class B common stock holders	12,000,000
Line of Credit
Debt Instrument [Line Items]
Bank line of credit facility amount		125,000,000
Bank line of credit facility, outstanding		92,900,000
Remaining borrowing capacity		29,600,000
Line of credit, conditional increase in maximum borrowing capacity		$ 50,000,000.0
Fixed charge ratio, minimum (percent)		1.25
Leverage ratio, maximum (percent)		2.25
Aggregate annual capital expenditures allowed		$ 20,000,000
Maximum exception to allowed restricted payments for the new share repurchase program	50,000,000.0
Line of Credit | Minimum
Debt Instrument [Line Items]
Aggregate annual capital expenditures allowed	20,000,000
Annual dividend payments allowed	25,000,000.0
Line of Credit | Maximum
Debt Instrument [Line Items]
Aggregate annual capital expenditures allowed	25,000,000
Annual dividend payments allowed	$ 35,000,000.0
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%
Letter of Credit
Debt Instrument [Line Items]
Remaining borrowing capacity		$ 2,500,000
Capital Lease Obligations
Debt Instrument [Line Items]
Capital lease obligations		$ 7,300,000	$ 3,800,000