Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2014
Income Tax [Line Items]
Unrecognized tax benefits	$ 582
Unrecognized tax benefits that would impact income tax expense and the effective tax rate	427
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	(158)
Income Tax Examination, Penalties and Interest Expense	4
Accrued interest and penalties on unrecognized tax benefits	161
Canada Revenue Agency Assessments [Member]
Income Tax [Line Items]
Tax assessments received	261
Internal Revenue Service (IRS) [Member]
Income Tax [Line Items]
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority	21
Tax Adjustments, Settlements, and Unusual Provisions	$ (17)