Postretirement Benefits (Schedule Of Changes In Benefit Obligations For Postretirement Welfare Plans) (Details) (Defined Postretirement Benefit Plans [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Defined Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period	$ 152	$ 154
Service cost	2	1	1
Interest cost	7	8	9
Actuarial loss (gain)	(4)	2
Benefit payments	(13)	(13)
Benefit obligation at end of period	$ 144	$ 152	$ 154

Postretirement Benefits (Schedule Of Changes In Benefit Obligations For Postretirement Welfare Plans) (Details) (Defined Postretirement Benefit Plans [Member], USD $)
In Millions, unless otherwise specified

12 Months Ended

Mar. 31, 2012

Mar. 31, 2011

Mar. 31, 2010

Defined Postretirement Benefit Plans [Member]

Defined Benefit Plan Disclosure [Line Items]

Benefit obligation at beginning of period

$ 152

$ 154

Service cost

Interest cost

Actuarial loss (gain)

(4)

Benefit payments

(13)

Benefit obligation at end of period

$ 144

$ 152

$ 154

- Definition

The net increase or decrease of changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Gain or Loss

-URI http://asc.fasb.org/extlink&oid=6514294

-Topic 715

-SubTopic 30

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Gain or Loss

-URI http://asc.fasb.org/extlink&oid=6749293

-Topic 715

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 106

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 264

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

- Definition

1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Projected Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6522206

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Accumulated Postretirement Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6503904

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 6

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 715

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Benefits

-URI http://asc.fasb.org/extlink&oid=6506309

-Topic 715

-SubTopic 60

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-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS106-2

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Benefits

-URI http://asc.fasb.org/extlink&oid=6506267

-Topic 715

-SubTopic 30

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Postretirement Benefits

-URI http://asc.fasb.org/extlink&oid=6521376

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-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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+ Details

- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References+ Details

- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

+ Details