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Income Taxes (Details 1)
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation      
Statutory U.S. federal income tax rate 35.00% 35.00% 35.00%
State income taxes, net of federal tax benefit 5.50% 5.60% 5.20%
State tax credits, net of federal effect (1.20%) (1.70%) (1.50%)
Non deductible expenses 0.20% 0.40% (0.10%)
Other (1.50%) (0.60%) 1.10%
Effective income tax rate 38.00% 38.70% 39.70%