Debt (Schedule of Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, due within one year	$ 8,976	$ 11,037
Long-term debt, due after one year	35,217	33,189
Total long-term debt	44,193	44,226
Derivative instrument converting variable-rate debt into fixed-rate obligations	0	3,400
Derivative instrument converting fixed rate debt into variable rate debt	0	3,800
Revolving credit facilities
Debt Instrument [Line Items]
Long-term debt, due after one year	6,700	8,100
Unsecured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	1,663	3,540
Long-term debt, due after one year	10,444	12,091
Total long-term debt	12,107	15,631
Secured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	7,313	7,497
Long-term debt, due after one year	24,773	21,098
Total long-term debt	$ 32,086	$ 28,595