Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Tax [Line Items]
Income Tax Expense (Benefit)	$ 222	$ 213	$ 404	$ 387
Income tax benefit (expense) discrete items	6	(6)	18	0
Unrecognized tax benefits	580		580
Unrecognized tax benefits, portion reducing income tax expense and ETR	424		424
Unrecognized tax benefits, potential reductions	158		158
Income Tax Examination, Penalties and Interest Expense			7
Accrued interest and penalties on unrecognized tax benefits	161		161
Canada Revenue Agency Assessments [Member]
Income Tax [Line Items]
Tax assessments received			261
Internal Revenue Service (IRS) [Member]
Income Tax [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions (Benefits)	(17)
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority	$ 21		$ 21