Borrowing Arrangements - Additional Information (Detail) - Revolving Credit and Security Agreement [Member] £ in Thousands		6 Months Ended
	May 20, 2016 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 GBP (£)	Dec. 31, 2017 USD ($)
Line Of Credit Facility [Line Items]
Initial term of credit facility	5 years
Line of credit facility maturity date	May 31, 2021
Agreement borrowing capacity	$ 100,000,000
Additional borrowing capacity	$ 50,000,000
Long term debt		$ 40,320,000		$ 20,349,000
Line of credit, remaining borrowing capacity		$ 37,000,000
Average interest rate		2.64%
Minimum [Member]
Line Of Credit Facility [Line Items]
Commitment fee payable percentage	0.25%
Maximum [Member]
Line Of Credit Facility [Line Items]
Commitment fee payable percentage	0.375%
Fixed charge coverage ratio		1.00	1.00
London Interbank Offered Rate (LIBOR) [Member]
Line Of Credit Facility [Line Items]
Debt instrument description of interest rate		LIBOR plus an applicable margin ranging between 1.25% to 1.75%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Debt instrument basis spread	1.25%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Debt instrument basis spread	1.75%
Base Rate [Member]
Line Of Credit Facility [Line Items]
Debt instrument description of interest rate		the base rate plus an applicable margin ranging between 0.25% to 0.75%
Base Rate [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Debt instrument basis spread	0.25%
Base Rate [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Debt instrument basis spread	0.75%
Letter of Credit [Member]
Line Of Credit Facility [Line Items]
Agreement borrowing capacity	$ 40,000,000
European Credit Facility [Member]
Line Of Credit Facility [Line Items]
Agreement borrowing capacity	15,000,000
Long term debt | £			£ 1,000
Canadian Credit Facility [Member]
Line Of Credit Facility [Line Items]
Agreement borrowing capacity	$ 15,000,000