Income Tax (Reconciliation of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit)		$ (1,470)	$ 693	$ 1,590
Operating Expenses		13,621	13,749	14,777
Reconciliation of Unrecognized Tax Benefits
Balance at January 1,		1,146	1,259	719
Additions for tax positions of prior years (1)		70	24	574
Reductions for tax positions of prior years	$ (792)	(101)	(112)	(24)
Additions for tax positions of current year		33	23	24
Reductions for tax positions of current year		(3)	0	0
Settlements with tax authorities		(43)	(48)	(34)
Balance at December 31, (1)		1,102	1,146	1,259
Unrecognized tax benefits that, if recognized, would impact the effective rate		$ 1,073	$ 1,112	1,215
Possible Change in Tax Assessment [Member]
Income Tax Expense (Benefit)	557			$ 557
Operating Expenses	362
Operating Expenses Net	$ 235