Postretirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payments, Year Two		$ 190
Expected amortization of actuarial loss from stockholders' equity in 2015		(31)
Other postretirement plan expected future benefit payments, 2020 through 2024		413
Expected contributions for postretirement benefit plans in 2015		67
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	6	8	6
Net prior service credits for post retirement plans	1	1	1
Net actuarial gains (loss) for changes in benefit obligations		2	7	3
Expected amortization of actuarial loss from stockholders' equity in 2015		4
Amortization of actuarial loss from stockholders' equity		1
Defined Benefit Plan Expected Future Benefit Payments Five Fiscal Years Beginning 2015		10
Other postretirement plan expected future benefit payments, 2020 through 2024		45
Expected contributions for postretirement benefit plans in 2015			$ 11
Weighted-average discount rates used to estimate postretirement welfare benefit expenses		3.84%	4.44%	5.09%
Weighted-average discount rates for actuarial present value of benefit obligations	3.84%	4.08%	3.84%	4.44%
Actuarial Gain Loss Postretirement Welfare Benefit Plan Amortization Period	3 years
Prescription Drugs [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate		7.00%
Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation			7.50%
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%	5.25%
Pre-65 [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		7.50%
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			7.50%
Post-65 [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		7.00%
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			7.25%