Income Taxes - Summary of Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets:
Workers' compensation claims liabilities	$ 11,477	$ 11,190
MCC accrual	4,561	3,532
Safety incentives payable	7,779	5,851
Allowance for doubtful accounts	31	109
Deferred compensation		219
Equity based compensation	991	719
Tax effect of unrealized losses, net	(17)	(3)
Alternative minimum tax credit carryforward	1,648	1,831
State credit carryforward	868	712
Other	765	436
Deferred tax assets, gross total	28,103	24,596
Less valuation allowance	216	216
Deferred tax assets, net total	27,887	24,380
Deferred income tax liabilities:
Tax depreciation in excess of book depreciation	(5,319)	(4,243)
Tax amortization of goodwill	(13,191)	(12,395)
Other	(7)	(57)
Deferred Tax Liabilities, gross total	(18,517)	(16,695)
Net deferred income tax assets	$ 9,370	$ 7,685