Schedule of Valuation and Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	$ 2,474	$ 17,009	$ 19,671
Charged to costs and expenses	411	(14,299)	(112)
Deductions	154	236	2,550
Balance at end of period	2,731	2,474	17,009
Credit losses [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	570	984	1,757
Charged to costs and expenses	2	(115)	(211)
Deductions	137	299	562
Balance at end of period	435	570	984
Doubtful receivables [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	110	116	276
Charged to costs and expenses	13	(69)	(98)
Deductions	17	(63)	62
Balance at end of period	106	110	116
Inventories (primarily service part obsolescence) [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	249	245	242
Charged to costs and expenses	18	4	3
Deductions	0	0	0
Balance at end of period	267	249	245
Deferred tax assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	1,545	15,664	17,396
Charged to costs and expenses	378	(14,119)	194
Deductions	0	0	1,926
Balance at end of period	1,923	1,545	15,664
Valuation Allowance Of Deferred Tax Assets Recognized In Accumulated Other Comprehensive Income Loss [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to costs and expenses	0	0	572
Valuation Allowance of Deferred Tax Assets Recognized in Income Statement [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to costs and expenses	$ 378	$ (14,100)	$ (378)