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Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2012
Income Tax Disclosure [Abstract]  
Factors Causing Variation in Effective Tax Rate From US Federal Statutory Rate
The differences that caused our effective income tax rates to vary from the 35% U.S. federal statutory rate for income taxes were as follows:
 
Nine Months Ended
 
September 30,
 
2012
 
2011
 
(in millions)
Income tax benefit at the federal statutory rate
$
1,013

 
$
504

Effect of:
 
 
 
State income taxes, net of federal income tax effect
104

 
(7
)
Change in valuation allowance
(1,210
)
 
(654
)
Other, net
(17
)
 
9

Income tax expense
$
(110
)
 
$
(148
)
Effective income tax rate
(3.8
)%
 
(10.3
)%