Postretirement Benefits (Narrative) (Details) (Defined Postretirement Benefit [Member], USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gains (losses) for post retirement plans	$ 6	$ 6	$ (2)
Net prior service credits for post retirement plans	1	1	2
Net actuarial gains (loss) for changes in benefit obligations		7	3	(6)
Expected amortization of actuarial loss from stockholders' equity in 2014		2
Amortization of actuarial loss from stockholders' equity		2	1	4
Other postretirement plan expected future benefit payments, 2014	11	11
Other postretirement plan expected future benefit payments, 2015	11	11
Other postretirement plan expected future benefit payments, 2016	11	11
Other postretirement plan expected future benefit payments, 2017	11	11
Other postretirement plan expected future benefit payments, 2018	11	11
Other postretirement plan expected future benefit payments, 2019-2023	48	48
Expected contributions for postretirement benefit plans in 2014	$ 11	$ 11
Weighted-average discount rates used to estimate postretirement welfare benefit expenses		4.44%	5.09%	5.33%
Weighted-average discount rates for actuarial present value of benefit obligations	3.84%	3.84%	4.44%	5.09%
Actuarial Gain Loss Postretirement Welfare Benefit Plan Amortization Period	3 years
Prescription Drugs [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate		7.50%	8.00%
Medical [Member] | Pre-65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate		7.50%	7.50%
Medical [Member] | Post-65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate		7.25%	7.50%
Dental [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate		5.25%	5.50%