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Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Income Tax Rate Reconciliation [Line Items]      
Statutory U.S. federal income tax rate 35.00% 35.00% 35.00%
State income taxes, net of federal tax benefit 5.90% 5.50% 5.60%
State tax credits, net of federal effect (1.60%) (1.20%) (1.70%)
Non deductible expenses 0.20% 0.20% 0.40%
Other (0.70%) (1.50%) (0.60%)
Effective income tax rate 38.80% 38.00% 38.70%