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Stockholders' Equity - Repurchases of Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2016
Class of Stock [Line Items]
Stock repurchase program, remaining authorized amount	$ 6,800
Amount	$ 1,583	$ 1,749	$ 2,900
Average price per share	$ 70.64	$ 66.36	$ 72.95
2016 stock repurchase program
Class of Stock [Line Items]
Stock repurchase program, authorized amount					$ 12,000
2020 stock repurchase program
Class of Stock [Line Items]
Stock repurchase program, authorized amount				$ 5,000
Publicly announced program
Class of Stock [Line Items]
Shares repurchased and retired	22	26	40

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