Postretirement Benefits (Narrative) (Details) (Defined Postretirement Benefit Plans [Member], USD $)	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Net actuarial losses for post retirement plan	$ 2,000,000	$ 5,000,000
Net prior service credits for postretirement plans	2,000,000	2,000,000
Net actuarial gain (losses) for changes in benefit obligations	3,000,000	(6,000,000)	(51,000,000)
Expected amortization of actuarial loss from stockholders' equity in 2013	1,000,000
Amortization of actuarial loss from stockholders equity	1,000,000
Other postretirement plan, expected Medicare subsidy receipts	1,000,000
Expected contributions for postretirement benefit plans in 2013	13,000,000
Weighted-average discount rates used to estimate postretirement welfare benefit expenses	5.09%	5.33%	7.86%
Weighted-average discount rates for actuarial present value of benefit obligations	4.44%	5.09%	5.33%
Other postretirement plan expected future benefit payments, 2013	11,000,000
Other postretirement plan expected future benefit payments, 2014	11,000,000
Other postretirement plan expected future benefit payments, 2015	11,000,000
Other postretirement plan expected future benefit payments, 2016	11,000,000
Other postretirement plan expected future benefit payments, 2017	11,000,000
Other postretirement plan expected future benefit payments, 2018-2022	$ 50,000,000
Prescription Drugs [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed healthcare cost trend rate	8.00%	8.50%
Medical [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed healthcare cost trend rate	7.50%	7.50%
Dental [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed healthcare cost trend rate	5.50%	5.80%