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Effective Tax Rate Differs from Statutory United States Federal Income Tax Rate (Detail)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Income Taxes [Line Items]      
Statutory Rate 35.00% 35.00% 35.00%
Increases in United States Federal Taxes      
State Income Taxes, Net of Federal Income Tax Benefit 2.50% 2.70% 2.70%
Other, Net 0.00% 0.30% 0.30%
Effective Tax Rate 37.50% 38.00% 38.00%