Postretirement Benefit Plans (Funded Status and Amounts Recognized) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued postretirement benefits	$ (748)	$ (621)
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status at end of period	(630)	(509)
Benefit obligation of unfunded plans	(1,863)	(1,672)	(1,581)
Other non- current assets	1	0
Accrued expenses and other current liabilities	(12)	(14)
Accrued postretirement benefits	(619)	(495)
As included in our consolidated balance sheets within Total assets / (Total liabilities)	(630)	(509)
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status at end of period	(139)	(135)
Benefit obligation of unfunded plans	(139)	(135)	(234)
Other non- current assets	0	0
Accrued expenses and other current liabilities	(10)	(9)
Accrued postretirement benefits	(129)	(126)
As included in our consolidated balance sheets within Total assets / (Total liabilities)	(139)	(135)
Funded Plan [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status at end of period	(325)	(254)
Benefit obligation of unfunded plans	(1,558)	(1,417)
Funded Plan [Member] | Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status at end of period	0	0
Benefit obligation of unfunded plans	0	0
Unfunded Plan [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation of unfunded plans	(305)	(255)
Unfunded Plan [Member] | Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation of unfunded plans	$ (139)	$ (135)