Income Taxes (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Unrecognized tax benefits	$ 3,057	$ 3,106
Portion that, if recognized, would reduce tax expense and effective tax rate	2,233	2,253
Accrued interest on unrecognized tax benefits	570	559
Accrued penalties on unrecognized tax benefits	96	101
Lower Limit [Member]
Income Tax Disclosure [Line Items]
Portion that, if recognized, would reduce tax expense and effective tax rate	0	0
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months lower limit	0	0
Upper Limit [Member]
Income Tax Disclosure [Line Items]
Portion that, if recognized, would reduce tax expense and effective tax rate	450	350
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months upper limit	$ 500	$ 400