Debt - Short-term and Long-term Debt (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Commercial paper	$ 0		$ 100,000,000
Current maturities of long-term debt	0		131,000,000
Other, including capital leases	9,000,000		9,000,000
Total short-term debt	9,000,000		240,000,000
Long-term debt, carrying value	25,041,000,000		5,199,000,000
Repayment of long-term debt	131,000,000	$ 1,250,000,000
$250 million, 4.375% Notes due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 250,000,000		$ 250,000,000
Long-term debt, stated interest rate	4.375%		4.375%
Long-term debt, carrying value	$ 247,000,000		$ 249,000,000
$300 million, 5.125% Notes due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 300,000,000		$ 300,000,000
Long-term debt, stated interest rate	5.125%		5.125%
Long-term debt, carrying value	$ 298,000,000		$ 299,000,000
$1,000 million, Floating Rate Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value	1,000,000,000
Long-term debt, carrying value	996,000,000
$1,750 million, 3.2% Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value	$ 1,750,000,000
Long-term debt, stated interest rate	3.20%
Long-term debt, carrying value	$ 1,741,000,000
$78 million, 6.37% Notes due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 78,000,000		$ 78,000,000
Long-term debt, stated interest rate	6.37%		6.37%
Long-term debt, carrying value	$ 78,000,000		$ 78,000,000
$300 million, 4.5% Notes due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 300,000,000		$ 300,000,000
Long-term debt, stated interest rate	4.50%		4.50%
Long-term debt, carrying value	$ 296,000,000		$ 299,000,000
$1,000 million, Floating Rate Notes due 2021
Debt Instrument [Line Items]
Long-term debt, face value	1,000,000,000
Long-term debt, carrying value	995,000,000
$1,250 million, 3.4% Notes due 2021
Debt Instrument [Line Items]
Long-term debt, face value	$ 1,250,000,000
Long-term debt, stated interest rate	3.40%
Long-term debt, carrying value	$ 1,244,000,000
$750 million, 4% Notes due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 750,000,000		$ 750,000,000
Long-term debt, stated interest rate	4.00%		4.00%
Long-term debt, carrying value	$ 746,000,000		$ 745,000,000
$100 million, 7.65% Notes due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 100,000,000		$ 100,000,000
Long-term debt, stated interest rate	7.65%		7.65%
Long-term debt, carrying value	$ 100,000,000		$ 100,000,000
$17 million, 8.3% Notes due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 17,000,000		$ 17,000,000
Long-term debt, stated interest rate	8.30%		8.30%
Long-term debt, carrying value	$ 17,000,000		$ 17,000,000
$700 million, Floating Rate Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value	700,000,000
Long-term debt, carrying value	696,000,000
$3,100 million, 3.75% Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value	$ 3,100,000,000
Long-term debt, stated interest rate	3.75%
Long-term debt, carrying value	$ 3,082,000,000
$900 million, 3.25% Notes Due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 900,000,000		$ 900,000,000
Long-term debt, stated interest rate	3.25%		3.25%
Long-term debt, carrying value	$ 895,000,000		$ 894,000,000
$2,200 million, 4.125% Notes due 2025
Debt Instrument [Line Items]
Long-term debt, face value	$ 2,200,000,000
Long-term debt, stated interest rate	4.125%
Long-term debt, carrying value	$ 2,185,000,000
$600 million, 3.05% Notes due 2027
Debt Instrument [Line Items]
Long-term debt, face value	$ 600,000,000		$ 600,000,000
Long-term debt, stated interest rate	3.05%		3.05%
Long-term debt, carrying value	$ 594,000,000		$ 594,000,000
$259 million, 7.875% Debentures due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 259,000,000		$ 259,000,000
Long-term debt, stated interest rate	7.875%		7.875%
Long-term debt, carrying value	$ 259,000,000		$ 258,000,000
$3,800 million, 4.375% Notes due 2028
Debt Instrument [Line Items]
Long-term debt, face value	$ 3,800,000,000
Long-term debt, stated interest rate	4.375%
Long-term debt, carrying value	$ 3,771,000,000
$45 million, 8.3% Step Down Notes due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 45,000,000		$ 45,000,000
Long-term debt, stated interest rate	8.30%		8.30%
Long-term debt, carrying value	$ 45,000,000		$ 45,000,000
$191 million, 6.15% Notes due 2036 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 191,000,000		$ 191,000,000
Long-term debt, stated interest rate	6.15%		6.15%
Long-term debt, carrying value	$ 190,000,000		$ 190,000,000
$2,200 million, 4.8% Notes due 2038
Debt Instrument [Line Items]
Long-term debt, face value	$ 2,200,000,000
Long-term debt, stated interest rate	4.80%
Long-term debt, carrying value	$ 2,176,000,000
$121 million, 5.875% Notes due 2041 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 121,000,000		$ 121,000,000
Long-term debt, stated interest rate	5.875%		5.875%
Long-term debt, carrying value	$ 119,000,000		$ 119,000,000
$317 million, 5.375% Notes due 2042 [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 317,000,000		$ 317,000,000
Long-term debt, stated interest rate	5.375%		5.375%
Long-term debt, carrying value	$ 315,000,000		$ 315,000,000
$1,000 million, 3.875% Notes due 2047
Debt Instrument [Line Items]
Long-term debt, face value	$ 1,000,000,000		$ 1,000,000,000
Long-term debt, stated interest rate	3.875%		3.875%
Long-term debt, carrying value	$ 988,000,000		$ 988,000,000
$3,000 million, 4.9% Notes due 2048
Debt Instrument [Line Items]
Long-term debt, face value	$ 3,000,000,000
Long-term debt, stated interest rate	4.90%
Long-term debt, carrying value	$ 2,961,000,000
Other, including capital leases [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 7,000,000		$ 9,000,000