Long-Term Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Debt Instrument [Line Items]
Long-term debt instruments	$ 30,234,000,000	$ 32,490,000,000
Debt due within one year	(6,722,000,000)	(6,256,000,000)
Reclassified from short-term debt	10,000,000,000	9,000,000,000
Unamortized discounts and debt issuance costs	(35,000,000)	(41,000,000)
Total long-term debt	33,477,000,000	35,193,000,000
Long-term debt maturing 2018	6,722,000,000
Long-term debt maturing 2019	5,000,000,000
Long-term debt maturing 2020	5,054,000,000
Long-term debt maturing 2021	2,054,000,000
Long-term debt maturing 2022	4,050,000,000
Long-term debt maturing after 2022	7,354,000,000
Notes Payable | 3.191% notes due 2023
Debt Instrument [Line Items]
Long-term debt instruments	$ 2,250,000,000	2,250,000,000
Interest rate	3.191%
Notes Payable | 2.954% notes due 2026
Debt Instrument [Line Items]
Long-term debt instruments	$ 2,250,000,000	2,250,000,000
Interest rate	2.954%
Notes Payable | 1.718% notes due 2018
Debt Instrument [Line Items]
Long-term debt instruments	$ 2,000,000,000	2,000,000,000
Interest rate	1.718%
Notes Payable | 2.355% notes due 2022
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,750,000,000	2,000,000,000
Interest rate	2.355%
Notes Payable | 1.365% notes due 2018
Debt Instrument [Line Items]
Long-term debt instruments	$ 2,000,000,000	1,750,000,000
Interest rate	1.365%
Notes Payable | 1.961% notes due 2020
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,750,000,000	1,750,000,000
Interest rate	1.961%
Notes Payable | 4.950% notes due 2019
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,500,000,000	1,500,000,000
Interest rate	4.95%
Notes Payable | 1.561% notes due 2019
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,350,000,000	1,350,000,000
Interest rate	1.561%
Notes Payable | 2.100% notes due 2021
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,350,000,000	1,350,000,000
Interest rate	2.10%
Notes Payable | 1.790% notes due 2018
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,250,000,000	1,250,000,000
Interest rate	1.79%
Notes Payable | 2.419% notes due 2020
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,250,000,000	1,250,000,000
Interest rate	2.419%
Notes Payable | 2.427% notes due 2020
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,000,000,000	1,000,000,000
Interest rate	2.427%
Notes Payable | 2.895% notes due 2024
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,000,000,000	0
Interest rate	2.895%
Notes Payable | 2.193% notes due 2019
Debt Instrument [Line Items]
Long-term debt instruments	$ 750,000,000	750,000,000
Interest rate	2.193%
Notes Payable | 2.566% notes due 2023
Debt Instrument [Line Items]
Long-term debt instruments	$ 750,000,000	750,000,000
Interest rate	2.566%
Notes Payable | 3.326% notes due 2025
Debt Instrument [Line Items]
Long-term debt instruments	$ 750,000,000	750,000,000
Interest rate	3.326%
Notes Payable | 2.498% notes due 2022
Debt Instrument [Line Items]
Long-term debt instruments	$ 700,000,000	0
Interest rate	2.498%
Notes Payable | 2.411% notes due 2022
Debt Instrument [Line Items]
Long-term debt instruments	$ 700,000,000	700,000,000
Interest rate	2.411%
Notes Payable | 1.991% notes due 2020
Debt Instrument [Line Items]
Long-term debt instruments	$ 600,000,000	0
Interest rate	1.991%
Notes Payable | 1.686% notes due 2019
Debt Instrument [Line Items]
Long-term debt instruments	$ 550,000,000	0
Interest rate	1.686%
Notes Payable | Medium-term notes, maturing from 2021 to 2038 (6.283%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 38,000,000	38,000,000
Notes Payable | 1.104% notes due 2017
Debt Instrument [Line Items]
Long-term debt instruments	$ 0	2,000,000,000
Interest rate	1.104%
Notes Payable | 1.345% notes due 2017
Debt Instrument [Line Items]
Long-term debt instruments	$ 0	1,100,000,000
Interest rate	1.345%
Notes Payable | 1.344% notes due 2017
Debt Instrument [Line Items]
Long-term debt instruments	$ 0	1,000,000,000
Interest rate	1.344%
Floating Rate Notes | Floating rate notes due 2018 (1.833%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 1,650,000,000	1,650,000,000
Interest rate	1.833%
Floating Rate Notes | Floating rate notes due 2019 (1.684%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 850,000,000	400,000,000
Interest rate	1.684%
Floating Rate Notes | Floating rate notes due 2021 (2.109%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 650,000,000	650,000,000
Interest rate	2.109%
Floating Rate Notes | Floating rate notes due 2022 (1.994%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 650,000,000	350,000,000
Interest rate	1.994%
Floating Rate Notes | Floating rate notes due 2020 (1.697%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 400,000,000	0
Interest rate	1.697%
Floating Rate Notes | Floating rate notes due 2017
Debt Instrument [Line Items]
Long-term debt instruments	$ 0	2,050,000,000
Bank Loans | Amortizing bank loan due 2018 (2.179%)
Debt Instrument [Line Items]
Long-term debt instruments	$ 72,000,000	178,000,000
Interest rate	2.179%
Debentures | 8.625% debentures due 2032
Debt Instrument [Line Items]
Long-term debt instruments	$ 147,000,000	147,000,000
Interest rate	8.625%
Debentures | 8.625% debentures due 2031
Debt Instrument [Line Items]
Long-term debt instruments	$ 108,000,000	108,000,000
Interest rate	8.625%
Debentures | 8.000% debentures due 2032
Debt Instrument [Line Items]
Long-term debt instruments	$ 75,000,000	75,000,000
Interest rate	8.00%
Debentures | 9.750% debentures due 2020
Debt Instrument [Line Items]
Long-term debt instruments	$ 54,000,000	54,000,000
Interest rate	9.75%
Debentures | 8.875% debentures due 2021
Debt Instrument [Line Items]
Interest rate	8.875%
Medium-term Notes | 8.875% debentures due 2021
Debt Instrument [Line Items]
Long-term debt instruments	$ 40,000,000	$ 40,000,000
Medium-term Notes | Medium-term notes, maturing from 2021 to 2038 (6.283%)
Debt Instrument [Line Items]
Interest rate	6.283%
Corporate Bond Securities
Debt Instrument [Line Items]
Debt issued			$ 4,000,000,000