Summary of Significant Accounting Policies Significant Accounting Policies - Narrative (Details) person in Millions, patient in Millions, Plans in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) person Plans building clinic patient Segment state	Mar. 31, 2018 USD ($)
Accounting Policies - Narrative [Abstract]
Number of Stores | building	9,900
Number of Walk In Medical Clinics | clinic	1,100
Number of Pharmacy Plan Members | Plans	94
Number of Patients Served per Year | patient	1
Number of People Served through Health Insurance Products and Related Services | person	38
Number of Reportable Segments | Segment	4
Related Party Transaction, Expenses from Transactions with Related Party	$ 10	$ 22
Number of States in which Entity Operates | state	4
Related Party Transaction, Other Revenues from Transactions with Related Party	$ 25	$ 35