Debt - Long-tem Debt Issued (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Amount net of discount and fees		$ 39,523,000,000	$ 5,199,000,000
Commercial paper		1,500,000,000	100,000,000
Net proceeds on issuance of long-term debt	$ 20,000,000,000	22,856,000,000	1,581,000,000	$ 0
$1,000 million, Floating Rate Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value		1,000,000,000
Amount net of discount and fees		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%
Amount net of discount and fees		$ 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
Amount net of discount and fees		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%
Amount net of discount and fees		$ 1,245,000,000	0
$700 million, Floating Rate Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value		700,000,000
Amount net of discount and fees		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%
Amount net of discount and fees		$ 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%
Amount net of discount and fees		$ 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%
Amount net of discount and fees		$ 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%
Amount net of discount and fees		$ 2,178,000,000	0
$3,000 million Floating Rate Term Loan due 2021
Debt Instrument [Line Items]
Long-term debt, face value		3,000,000,000
Amount net of discount and fees		2,997,000,000	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%
Amount net of discount and fees		$ 2,964,000,000	0
Notes Issued Third Quarter 2017 [Member]
Debt Instrument [Line Items]
Long-term debt, issuance date		Sep. 14, 2017
Long-term debt, face value		$ 1,600,000,000
$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%
Amount net of discount and fees		$ 595,000,000	$ 594,000,000
$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%
Amount net of discount and fees		$ 988,000,000	$ 988,000,000