Income Taxes (Federal Statutory) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal tax at statutory rate		$ 5,363	$ 5,024	$ 4,680
State income taxes, net of federal benefit		298	373	326
Foreign income taxes, net of federal credit		70	65	13
Nontaxable income attributable to noncontrolling interests		(45)	(128)	(69)
Other adjustments		(229)	(67)	(79)
Excess tax benefits recognized on share-based compensation		(297)	0	0
2017 federal tax reform enactment	$ (12,700)	(12,679)	0	0
Income tax expense (benefit)		(7,578)	5,308	4,959
Adjustments to uncertain and effectively settled tax positions, net [Member]
Income Taxes [Line Items]
Other adjustments		(62)	24	15
Accrued interest on uncertain and effectively settled tax positions, net [Member]
Income Taxes [Line Items]
Other adjustments		$ 3	$ 17	$ 73