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Income Taxes (Tables)
 3 Months Ended
Jun. 30, 2017
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 differ from the 35% U.S. federal statutory rate for income taxes were as follows:
 
Three Months Ended
June 30,
 
2017
 
2016
 
(in millions)
Income tax (expense) benefit at the federal statutory rate
$
(174
)
 
$
86

Effect of:
 
 
 
State income taxes, net of federal income tax effect
(22
)
 
3

State law changes, net of federal income tax effect
3

 
—

Increase deferred tax liability for business activity changes
(65
)
 
—

Credit for increasing research activities
4

 
—

Change in federal and state valuation allowance
(33
)
 
(142
)
Other, net
(5
)
 
(3
)
Income tax expense
$
(292
)
 
$
(56
)
Effective income tax rate
58.6
%
 
(22.8
)%