Debt - Short-term and Long-term Debt (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Commercial paper			$ 1,500,000,000	$ 100,000,000
Current maturities of long-term debt			0	131,000,000
Other, including capital leases			117,000,000	9,000,000
Total short-term debt			2,955,000,000	240,000,000
Long-term debt, carrying value			39,523,000,000	5,199,000,000
Repayment of long-term debt	$ 131,000,000	$ 250,000,000	131,000,000	1,250,000,000	$ 0
$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			997,000,000	0
$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			$ 1,743,000,000	0
$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			996,000,000	0
$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			$ 1,245,000,000	0
$3,000 million Floating Rate Term Loan due 2021
Debt Instrument [Line Items]
Long-term debt, face value			3,000,000,000
Long-term debt, carrying value			2,997,000,000	0
$700 million, Floating Rate Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value			700,000,000
Long-term debt, carrying value			697,000,000	0
$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			$ 3,085,000,000	0
$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			$ 2,187,000,000	0
$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			$ 3,774,000,000	0
$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			$ 2,178,000,000	0
$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			$ 2,964,000,000	0
$500 million, 4.125% Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value			$ 500,000,000
Long-term debt, stated interest rate			4.125%
Long-term debt, carrying value			$ 506,000,000	0
$500 million, 2.600% Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value			$ 500,000,000
Long-term debt, stated interest rate			2.60%
Long-term debt, carrying value			$ 493,000,000	0
$400 million, Floating Rate Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value			400,000,000
Long-term debt, carrying value			399,000,000	0
$500 million, 3.300% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term debt, face value			$ 500,000,000
Long-term debt, stated interest rate			3.30%
Long-term debt, carrying value			$ 499,000,000	0
$1,250 million, 4.750% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,250,000,000
Long-term debt, stated interest rate			4.75%
Long-term debt, carrying value			$ 1,285,000,000	0
$1,000 million, 3.900% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,000,000,000
Long-term debt, stated interest rate			3.90%
Long-term debt, carrying value			$ 998,000,000	0
$500 million, 3.050% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, face value			$ 500,000,000
Long-term debt, stated interest rate			3.05%
Long-term debt, carrying value			$ 481,000,000	0
$1,000 million, 3.000% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,000,000,000
Long-term debt, stated interest rate			3.30%
Long-term debt, carrying value			$ 959,000,000	0
$1,000 million, 3.500% Senior Notes due 2024
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,000,000,000
Long-term debt, stated interest rate			3.50%
Long-term debt, carrying value			$ 966,000,000	0
$1,500 million, 4.500% Senior Notes due 2026
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,500,000,000
Long-term debt, stated interest rate			4.50%
Long-term debt, carrying value			$ 1,508,000,000	0
$1,500 million, 3.400% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,500,000,000
Long-term debt, stated interest rate			3.40%
Long-term debt, carrying value			$ 1,386,000,000	0
$449 million, 6.125% Senior Notes due 2041
Debt Instrument [Line Items]
Long-term debt, face value			$ 449,000,000
Long-term debt, stated interest rate			6.125%
Long-term debt, carrying value			$ 493,000,000	0
$1,500 million, 4.800% Senior Notes due 2046
Debt Instrument [Line Items]
Long-term debt, face value			$ 1,500,000,000
Long-term debt, stated interest rate			4.80%
Long-term debt, carrying value			$ 1,465,000,000	0
$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			$ 248,000,000	$ 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
$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			$ 297,000,000	$ 299,000,000
$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
$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
$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			$ 595,000,000	$ 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
$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
$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
Other, including capital leases [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value			$ 32,000,000	$ 9,000,000