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Income Taxes (Tables)
 3 Months Ended
Mar. 31, 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:
 
Three Months Ended
 
March 31,
 
2013
 
2012
 
(in millions)
Income tax benefit at the federal statutory rate
$
212

 
$
289

Effect of:
 
 
 
State income taxes, net of federal income tax effect
16

 
25

Change in valuation allowance
(265
)
 
(348
)
Other, net
(1
)
 
(3
)
Income tax expense
$
(38
)
 
$
(37
)
Effective income tax rate
(6.3
)%
 
(4.5
)%