Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2012
Income Tax [Line Items]
Unrecognized tax benefits	$ 519
Unrecognized tax benefits that would impact income tax expense and the effective tax rate	393
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions	82
Potential reduction of unrecognized benefits due to audit resolutions and expiration of statutes of limitations	149
Accrued interest on unrecognized tax benefits	120
Income Tax Benefit Related to Accrued Interest Expense	20
Canada Revenue Agency Assessments [Member]
Income Tax [Line Items]
Tax assessments received relating to transfer pricing issues	169
Periods For Which Tax Assessments Were Received	2003 through 2007
Year for which assessment is appealed	2003
Periods For Which Tax Assessments Are Objected	2004 through 2007
Internal Revenue Service (IRS) [Member]
Income Tax [Line Items]
Tax assessments received relating to transfer pricing issues	$ 98
Periods For Which Tax Assessments Were Received	2003 through 2006
Periods Of Examination	2007 through 2009