Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 45	$ 47	$ 43
Interest cost	59	59	66
Expected return on assets	(61)	(67)	(69)
Amortization — losses	40	37	22
Amortization — prior service credit	(2)	(2)	(2)
Curtailment, settlement, and special termination losses (gains)	1	0	1
Net periodic benefit cost (income)	82	74	61
Proportionate share of non-consolidated affiliates’ pension costs	8	9	7
Total net periodic benefit costs (income) recognized	90	83	68
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	5	5
Interest cost	6	5	5
Expected return on assets	0	0	0
Amortization — losses	4	4	5
Amortization — prior service credit	2	(27)	(37)
Curtailment, settlement, and special termination losses (gains)	0	0	(1)
Net periodic benefit cost (income)	18	(13)	(23)
Proportionate share of non-consolidated affiliates’ pension costs	0	0	0
Total net periodic benefit costs (income) recognized	$ 18	$ (13)	$ (23)