Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Termination Benefits [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1	$ 111	$ 308	$ 49
Additions (reductions) charged to expense	20	160	342
Payments/ reductions	(69)	(357)	(83)
Balance at December 31	62	111	308
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1	487	429	194
Additions (reductions) charged to expense	128	76	251
Payments/ reductions	(9)	(18)	(16)
Balance at December 31	606	487	429
Deferred Income Tax Valuation Allowance [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1	16,069	15,412	16,292
Additions (reductions) charged to expense	2,681	1,810	1,440
Payments/ reductions	(2,176)	(1,153)	(2,320)
Balance at December 31	$ 16,574	$ 16,069	$ 15,412