Postretirement Benefit Plans (Postretirement Amounts Recognized in AOCI) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial losses	$ (375)	$ (444)
Prior service credit	4	9
Total postretirement amounts recognized in Accumulated other comprehensive (loss) income	(371)	(435)	$ (439)
Other Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial losses	(16)	(22)
Prior service credit	7	6
Total postretirement amounts recognized in Accumulated other comprehensive (loss) income	$ (9)	$ (16)	$ 18