Acquisitions (Details) $ / shares in Units, $ in Billions	Nov. 29, 2018 USD ($)	Mar. 31, 2019 building clinic	Nov. 28, 2018 USD ($) $ / shares
Business Acquisition [Line Items]
Number of Stores | building		9,900
Number of Walk In Medical Clinics | clinic		1,100
Aetna Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Per Share Exchange Ratio | $ / shares			$ 0.8378
Business Combination, Acquisition Value Per Share Acquired | $ / shares			212
Business Combination, Consideration Transferred | $	$ 70
Business Acquisition, Share Price | $ / shares			$ 145.00
Business Combination Acquisition Assigned Value Of Acquiree | $			$ 78
Long-term Debt | $			$ 45