Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Dec. 31, 2012	Dec. 31, 2012
Income Tax [Line Items]
Unrecognized tax benefits	$ 541	$ 541
Unrecognized tax benefits that would impact income tax expense and the effective tax rate	390	390
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	(170)	(170)
Accrued interest on unrecognized tax benefits	131	131
Income Tax Expense (Benefit), Before Any Tax Effect, Related to Accrued Interest Expense	10	(8)
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