Income Taxes (Details) (USD $) In Millions	3 Months Ended	6 Months Ended
Sep. 30, 2011	Sep. 30, 2011
Income Taxes
Unrecognized tax benefits	$ 654	$ 654
Unrecognized tax benefits that would impact income tax expense and the effective tax rate	433	433
Potential reduction of unrecognized benefits due to audit resolutions and expiration of statutes of limitations	163	163
Accrued interest on unrecognized tax benefits	142	142
Income tax expense, before any tax effect, related to interest	4	7
Canada Revenue Agency Assessments [Member]
Income Taxes
Tax assessments relating to transfer pricing issues		169
Internal Revenue Service (IRS) [Member]
Income Taxes
Tax assessments relating to transfer pricing issues		$ 98

Amount of accrued interest on unrecognized tax benefits.

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Income Taxes.

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- Definition

Reflects tax assessments from taxing authorities with which the Company disagrees.

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The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date.

+ References

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The gross amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns as of the balance sheet date.

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-Glossary Unrecognized Tax Benefit

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This element represents the amount of interest expense accrued as of the date of the statement of financial position for an underpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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