Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013
Income Tax [Line Items]
Unrecognized tax benefits	$ 567
Unrecognized tax benefits that would impact income tax expense and the effective tax rate	409
Potential reduction of unrecognized benefits due to audit resolutions and expiration of statutes of limitations	(173)
Accrued interest and penalties on unrecognized tax benefits	134
Income Tax Examination, Penalties and Interest Expense	3
Internal Revenue Service (IRS) [Member]
Income Tax [Line Items]
Tax assessments received	98
Earliest Tax Year Subject To Examination	2003
Internal Revenue Service (IRS) [Member] | 2003 through 2006 [Member] | Minimum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2003
Internal Revenue Service (IRS) [Member] | 2003 through 2006 [Member] | Maximum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2006
Internal Revenue Service (IRS) [Member] | 2007 through 2009 [Member] | Minimum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2007
Internal Revenue Service (IRS) [Member] | 2007 through 2009 [Member] | Maximum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2009
Canada Revenue Agency Assessments [Member]
Income Tax [Line Items]
Tax assessments received	$ 209
Canada Revenue Agency Assessments [Member] | 2003 through 2008 [Member] | Minimum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2003
Canada Revenue Agency Assessments [Member] | 2003 through 2008 [Member] | Maximum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2008
Canada Revenue Agency Assessments [Member] | 2003 [Member]
Income Tax [Line Items]
Year for which assessment is appealed	2003
Canada Revenue Agency Assessments [Member] | 2004 through 2007 [Member] | Minimum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2004
Canada Revenue Agency Assessments [Member] | 2004 through 2007 [Member] | Maximum [Member]
Income Tax [Line Items]
Periods For Which Tax Assessments Were Received	2008