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Income Taxes (Tables)
 6 Months Ended
Jun. 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:
 
Six Months Ended
 
June 30,
 
2012
 
2011
 
(in millions)
Income tax benefit at the federal statutory rate
$
761

 
$
403

Effect of:
 
 
 
State income taxes, net of federal income tax effect
87

 
(10
)
Change in valuation allowance
(902
)
 
(533
)
Other, net
(9
)
 
4

Income tax expense
$
(63
)
 
$
(136
)
Effective income tax rate
(2.9
)%
 
(11.8
)%