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Income Taxes - Summary of Deferred Income Tax Assets and Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Gross deferred income tax assets:    
Workers' compensation claims liabilities $ 6,985 $ 4,833
Safety incentives payable 3,529 3,422
Allowance for doubtful accounts 94 148
Deferred compensation 0 44
Tax effect of unrealized losses, net 959 936
Alternative minimum tax credit carryforward 0 0
State credit carryforward 805 805
State loss carryforward 0 0
Other 504 284
Deferred tax assets, gross total 12,876 10,472
Less valuation allowance 1,763 1,773
Deferred tax assets, net total 11,113 8,699
Gross deferred income tax liabilities:    
Tax depreciation in excess of book depreciation (2,793) (2,017)
Tax amortization of goodwill (10,178) (9,141)
Deferred Tax Liabilities, gross total (12,971) (11,158)
Net deferred income tax liabilities $ (1,858) $ (2,459)