Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 43	$ 48	$ 43
Interest cost	66	63	64
Expected return on assets	(69)	(67)	(64)
Amortization — losses	22	30	28
Amortization — prior service (credit)	(2)	(2)	(2)
Curtailment/settlement/special termination losses (gains)	1	1	1
Net periodic benefit cost (income)	61	73	70
Proportionate share of non-consolidated affiliates’ pension costs	7	7	5
Total net periodic benefit costs (income) recognized	68	80	75
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	8	10
Interest cost	5	7	10
Expected return on assets	0	0	0
Amortization — losses	5	7	3
Amortization — prior service (credit)	(37)	(24)	(1)
Curtailment/settlement/special termination losses (gains)	(1)	0	0
Net periodic benefit cost (income)	(23)	(2)	22
Proportionate share of non-consolidated affiliates’ pension costs	0	0	0
Total net periodic benefit costs (income) recognized	$ (23)	$ (2)	$ 22