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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Effective Tax Rate        
Income before income taxes $ 322 $ 354 $ 622 $ 713
Provision for income taxes at federal statutory rate of 35% 113 124 218 249
Increase (decrease) in income tax from:        
State tax - net of federal benefit 10 18 20 30
Property-related (10) (10) (20) (21)
Other (5) (4) (10) (7)
Total income tax expense 108 128 208 251
Effective tax rate (as a percent) 33.50% 36.20% 33.40% 35.20%
Federal statutory rate (as a percent) 35.00% 35.00% 35.00% 35.00%
IRS Examination
       
Increase (decrease) in income tax from:        
Estimated potential future federal tax payment as of the balance sheet date, including interest and penalties, based on a proposed adjustment $ 95   $ 95