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Postretirement Benefits Postretirement Benefits (Tables) (Defined Postretirement Benefit [Member])
 12 Months Ended
Mar. 31, 2013
Defined Postretirement Benefit [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Periodic Expense (Income) for Postretirement Welfare Benefits
 
Years Ended March 31,
(In millions)
2013
 
2012
 
2011
Service cost - benefits earned during the year
$
2

 
$
2

 
$
1

Interest cost on accumulated benefit obligation
6

 
7

 
8

Amortization of unrecognized actuarial gain and prior service costs
(2
)
 
(1
)
 
(4
)
Net periodic postretirement expense
$
6

 
$
8

 
$
5
Schedule of Changes in Benefit Obligations for Postretirement Welfare Plans
 
Years Ended March 31,
(In millions)
2013
 
2012
Benefit obligation at beginning of period
$
144

 
$
152

Service cost
2

 
2

Interest cost
6

 
7

Actuarial gain
(9
)
 
(4
)
Benefit payments
(12
)
 
(13
)
Benefit obligation at end of period
$
131

 
$
144