Debt - Long-tem Debt Issued (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amount net of discount and fees	$ 25,041,000,000	$ 5,199,000,000
Notes issued in 2018 in connection with Express Scripts Merger [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 20,000,000,000
Debt guarantee termination percentage	20.00%
Notes issued in 2018 in connection with Express Scripts Merger [Member] | Minimum [Member]
Debt Instrument [Line Items]
Long-term debt, term	18 months
Notes issued in 2018 in connection with Express Scripts Merger [Member] | Maximum [Member]
Debt Instrument [Line Items]
Long-term debt, term	30 years
Notes issued in 2018 with a special mandatory redemption [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 17,000,000,000
Debt instrument redemption price percentage	101.00%
$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	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%
Amount net of discount and fees	$ 1,741,000,000
$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	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%
Amount net of discount and fees	$ 1,244,000,000
$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	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%
Amount net of discount and fees	$ 3,082,000,000
$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%
Amount net of discount and fees	$ 2,185,000,000
$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%
Amount net of discount and fees	$ 3,771,000,000
$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%
Amount net of discount and fees	$ 2,176,000,000