Acquisition and Divestiture (Details) $ / shares in Units, $ in Millions	Nov. 28, 2018 USD ($) $ / shares	Sep. 30, 2019 USD ($) building clinic	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Long-term debt		$ 69,054	$ 73,429
Number of stores | building		9,900
Number of walk in medical clinics | clinic		1,100
Aetna Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%
Cash received by shareholders (in dollars per share) | $ / shares	$ 145.00
Per share exchange ratio (in shares) | $ / shares	0.8378
Acquisition value per share acquired (in dollars per share) | $ / shares	$ 212
Consideration transferred	$ 70,000
Acquisition assigned value of acquiree	78,000
Long-term debt	$ 45,000