Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	2.60%	4.30%
United States statutory tax rate	35.00%	35.00%
Unrecognized tax benefits, period increase (decrease)	$ (2)
Gross unrecognized tax benefits	295
Cash deposits to offset uncertain tax liabilities	46
Amount of unrecognized tax benefits that would affect the effective tax rate	102
The total amount of unrecognized tax benefits that, if recognized, would result in adjustments to deferred tax assets with corresponding adjustments to the valuation allowance	180
Unrecognized tax benefits income tax penalties and interest accrued increase (decrease)	1
Combined amount of accrued interest and penalties related to uncertain tax positions	24
Maximum
Income Tax Footnote Disclosure [Line Items]
Amount of unrecognized tax benefits for which it is reasonably possible that there will be a reduction within the next 12 months	$ 80