Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2012
Income Tax Contingency [Line Items]
Income tax expense	$ 815	$ 757	$ 587
Income tax benefit (expense) discrete items	33	(94)	29
Undistributed Earnings of Foreign Operations	4,916
Deferred Tax Liabilities, Undistributed Foreign Earnings	0
Unrecognized Tax Benefits	616	647	560		595
Unrecognized Tax Benefits that would Impact Income Tax Expense and the Effective Tax Rate	457
Potential Reduction of Unrecognized Benefits Due to Audit Resolutions and Expiration of Statutes of Limitations	137
Income Tax Expense (Benefit), Before Any Tax Effect, Related to Accrued Interest	(24)	48
Accrued interest and penalties on unrecognized tax benefits	122	179
Canada Revenue Agency Assessments [Member]
Income Tax Contingency [Line Items]
Income tax benefit (expense) discrete items		122		122
Internal Revenue Service (IRS) [Member]
Income Tax Contingency [Line Items]
Income tax benefit (expense) discrete items	55
Domestic Tax Authority [Member]
Income Tax Contingency [Line Items]
Federal, State and Foreign Operating Loss Carryforwards	44
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Federal, State and Foreign Operating Loss Carryforwards	1,930
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Federal, State and Foreign Operating Loss Carryforwards	$ 711