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Debt - Long-tem Debt Issued (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Amount net of discount and fees		$ 34,041		$ 39,523
Commercial paper		1,140		$ 1,500
Net proceeds on issuance of long-term debt		0	$ 19,884
Notes issued in 2018 in connection with Express Scripts Merger [Member]
Debt Instrument [Line Items]
Net proceeds on issuance of long-term debt	$ 20,000
$1,000 million, Floating Rate Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value		1,000
$1,750 million, 3.2% Notes due 2020
Debt Instrument [Line Items]
Long-term debt, face value		$ 1,750
Long-term debt, stated interest rate		3.20%
$1,000 million, Floating Rate Notes due 2021
Debt Instrument [Line Items]
Long-term debt, face value		$ 1,000
$1,250 million, 3.4% Notes due 2021
Debt Instrument [Line Items]
Long-term debt, face value		$ 1,250
Long-term debt, stated interest rate		3.40%
$700 million, Floating Rate Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value		$ 700
$3,100 million, 3.75% Notes due 2023
Debt Instrument [Line Items]
Long-term debt, face value		$ 3,100
Long-term debt, stated interest rate		3.75%
$2,200 million, 4.125% Notes due 2025
Debt Instrument [Line Items]
Long-term debt, face value		$ 2,200
Long-term debt, stated interest rate		4.125%
$3,800 million, 4.375% Notes due 2028
Debt Instrument [Line Items]
Long-term debt, face value		$ 3,800
Long-term debt, stated interest rate		4.375%
$2,200 million, 4.8% Notes due 2038
Debt Instrument [Line Items]
Long-term debt, face value		$ 2,200
Long-term debt, stated interest rate		4.80%
$3,000 million, 4.9% Notes due 2048
Debt Instrument [Line Items]
Long-term debt, face value		$ 3,000
Long-term debt, stated interest rate		4.90%