Postretirement Benefit Plans (Funded Status and Amounts Recognized) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued postretirement benefits	$ (799)	$ (820)
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status at end of period	(665)	(691)
Benefit obligation of unfunded plans	(1,865)	(1,868)	$ (1,863)
Other non- current assets	2	0
Accrued expenses and other current liabilities	(12)	(13)
Accrued postretirement benefits	(655)	(678)
As included in our consolidated balance sheets within Total assets / (Total liabilities)	(665)	(691)
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Funded status at end of period	(153)	(151)
Benefit obligation of unfunded plans	(153)	(151)	$ (139)
Other non- current assets	0	0
Accrued expenses and other current liabilities	(10)	(9)
Accrued postretirement benefits	(143)	(142)
As included in our consolidated balance sheets within Total assets / (Total liabilities)	(153)	(151)
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	(394)	(401)
Benefit obligation of unfunded plans	(1,594)	(1,578)
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	(271)	(290)
Unfunded Plan [Member] | Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit obligation of unfunded plans	$ (153)	$ (151)