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Income Tax (tables)
6 Months Ended
Jun. 30, 2012
Income Tax Expense Benefit Abstract  
Effective Tax Rate Reconciliation
  June 30,
 2012  2011 
Federal statutory tax rate 35.0%  35.0%
Increase (decrease):     
 Current state income tax, net of federal tax benefit 4.5%  4.5%
 Amortization of investment and energy tax credits (0.3)%  (0.4)%
 Differences required to be flowed-through by regulatory commissions 1.5%  1.6%
 Gains on company and trust-owned life insurance (0.7)%  (0.6)%
 Other - net 0.6%  0.3%
Effective income tax rate 40.6%  40.4%