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Postretirement Benefits (Tables) (Other Postretirement Benefit Plan, Defined Benefit [Member])
 12 Months Ended
Mar. 31, 2014
Other Postretirement Benefit Plan, Defined Benefit [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Periodic Expense (Income) for Postretirement Welfare Benefits
 
Years Ended March 31,
(In millions)
2014
 
2013
 
2012
Service cost - benefits earned during the year
$
2

 
$
2

 
$
2

Interest cost on accumulated benefit obligation
5

 
6

 
7

Amortization of unrecognized actuarial gain and prior service credit
(1
)
 
(2
)
 
(1
)
Curtailment gain
(2
)
 
—

 
—

Net periodic postretirement expense
$
4

 
$
6

 
$
8
Schedule of Changes in Benefit Obligations for Postretirement Welfare Plans
 
Years Ended March 31,
(In millions)
2014
 
2013
Benefit obligation at beginning of period
$
131

 
$
144

Service cost
2

 
2

Interest cost
5

 
6

Actuarial gain
(2
)
 
(9
)
Benefit payments
(15
)
 
(12
)
Curtailment gain
(2
)
 
—

Benefit obligation at end of period
$
119

 
$
131