Postretirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	$ 172
Expected amortization of actuarial loss from stockholders' equity in 2016	(47)
Other postretirement plan expected future benefit payments, 2021 through 2025	333
Expected contributions for postretirement benefit plans in 2016	54
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	75
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	94
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	66
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payments, Year Two		0
Net actuarial gains (losses) for post retirement plans	(1)	8
Net prior service credits for post retirement plans	1	1
Net actuarial gains (loss) for changes in benefit obligations	(9)	2	7
Expected amortization of actuarial loss from stockholders' equity in 2016	1
Amortization of actuarial loss from stockholders' equity	4
Defined Benefit Plan Expected Future Benefit Payments Five Fiscal Years Beginning 2016	10
Other postretirement plan expected future benefit payments, 2021 through 2025	44
Expected contributions for postretirement benefit plans in 2016	10
Weighted-average discount rates used to estimate postretirement welfare benefit expenses	4.07%	3.84%	4.44%
Weighted-average discount rates for actuarial present value of benefit obligations	3.61%	4.08%	3.84%
Actuarial Gain Loss Postretirement Welfare Benefit Plan Amortization Period	3 years
Defined Benefit Plan, Expected Future Benefit Payments, Year Three		0
Defined Benefit Plan, Expected Future Benefit Payments, Year Four		0
Defined Benefit Plan, Expected Future Benefit Payments, Year Five		$ 0
Prescription Drugs [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate	6.75%	7.00%
Dental [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation	5.00%	5.00%
Pre-65 [Member] | Medical [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation	7.25%	7.50%
Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation, Age Threshold	pre-65
Post-65 [Member] | Medical [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation	6.75%	7.00%
Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation, Age Threshold	post-65