Revolving Credit Facility and Long-Term Debt - Additional Information (Detail) - Agreement [Member] - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of credit facility commitment fee description				The Agreement also provides for an unused commitment fee of 0.35% per year through December 31, 2016 and 0.375% per year thereafter on the average daily unused amount of the revolving credit facility, and a fee of 1.75% of the face amount of each letter of credit reserved under the line of credit and 0.95% on standalone, fully secured letters of credit.
Restricted money market funds and restricted certificates of deposit, current				$ 5,300,000
Minimum [Member]
Line of Credit Facility [Line Items]
EBITDA				$ 22,000,000
Ratio of restricted and unrestricted cash and marketable securities to workers’ compensation and safety incentive liabilities				100.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Prohibition on incurring additional indebtedness without the prior approval of the Bank in purchase money financing				$ 1,000,000
Scenario Forecast [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
EBITDA	$ 25,000,000	$ 25,000,000	$ 25,000,000
Ratio of restricted and unrestricted cash and marketable securities to workers’ compensation and safety incentive liabilities	100.00%	100.00%	100.00%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Unused commitment fee on unused amount during period					0.35%
Unused commitment fee on unused amount after amendment				0.375%
Outstanding balance on term loan and revolving credit facility				$ 0	$ 0
Credit facility, expiration date				Jul. 01, 2018
Revolving Credit Facility [Member] | Daily Floating Rate [Member]
Line of Credit Facility [Line Items]
Term loan from Bank, Interest rate description				A daily floating rate of one month LIBOR plus 1.75%
LIBOR plus rate				1.75%
Revolving Credit Facility [Member] | Fixed Rate [Member]
Line of Credit Facility [Line Items]
Term loan from Bank, Interest rate description				A fixed rate of LIBOR plus 1.75%
LIBOR plus rate				1.75%
Revolving Credit Facility [Member] | Wells Fargo [Member]
Line of Credit Facility [Line Items]
Credit facility borrowing capacity				$ 25,000,000
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility borrowing used				5,300,000
Standby Letters of Credit [Member] | Wells Fargo [Member]
Line of Credit Facility [Line Items]
Credit facility borrowing capacity				$ 6,000,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Unused commitment fee on unused amount				1.75%
Fully Secured Letter of Credit [Member]
Line of Credit Facility [Line Items]
Unused commitment fee on unused amount				0.95%
Fully Secured Letter of Credit [Member] | Wells Fargo [Member]
Line of Credit Facility [Line Items]
Credit facility borrowing capacity				$ 5,000,000
Term Loan [Member]
Line of Credit Facility [Line Items]
Term loan from Bank, Interest rate description				One month LIBOR plus 2.25%
Term loan with principal bank				$ 4,600,000	$ 4,600,000
Payment of monthly principal of term loan				$ 18,375
Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
LIBOR plus rate				2.25%