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Income Taxes (Tables)
 6 Months Ended
Jun. 30, 2013
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,
 
2013
 
2012
 
(in millions)
Income tax benefit at the federal statutory rate
$
751

 
$
761

Effect of:
 
 
 
State income taxes, net of federal income tax effect
63

 
87

Change in valuation allowance
(886
)
 
(902
)
Other, net
(21
)
 
(9
)
Income tax expense
$
(93
)
 
$
(63
)
Effective income tax rate
(4.3
)%
 
(2.9
)%