Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument
Medium-term notes, maximum remaining maturity	9 years
Long-term debt	$ 22,106	$ 20,537
Long-term debt outstanding maturity:
2018	6,188
2019	5,681
2020	4,290
2021	1,740
2022	2,048
Thereafter	2,159
Total	22,106	20,537
Medium-term notes, callable	148
Debt exchange, original medium term notes amount		381
Debt exchange, new medium term notes amount		$ 366
Debt exchange, new medium term notes interest rate (as a percent)		1.93%
Debt exchange, new medium term notes due date (year)		2021
Debt exchange, cash paid		$ 15
Debt exchange premium	0	33	$ 0
Medium-term notes
Debt Instrument
Long-term debt issued	7,290
Long-term debt issued at fixed interest rates	4,890
Long-term debt issued at floating interest rates	2,400
Long term debt, gross	21,362	19,731
Unamortized discount and debt issuance costs	(59)	(64)
Long-term debt	$ 21,303	$ 19,667
Long-term debt, Avg. Rate	2.50%	2.40%
Long-term debt outstanding maturity:
Total	$ 21,303	$ 19,667
Bank borrowings
Debt Instrument
Long-term debt	$ 803	$ 870
Long-term debt, Avg. Rate	5.00%	4.50%
Long-term debt outstanding maturity:
Total	$ 803	$ 870