Significant Accounting Policies - Narrative (Details) person in Millions, patient in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) person building clinic patient state	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) person building clinic patient Segment state	Sep. 30, 2018 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of stores | building	9,900		9,900
Number of walk in medical clinics | clinic	1,100		1,100
Number of pharmacy plan members | person	102		102
Number of patients served per year | patient	1		1
Number of people served through health insurance products and related services | person	38		38
Number of reportable segments | Segment			4
Related party transaction, expenses from transactions with related party	$ 14	$ 4	$ 26	$ 34
Related party transaction, other revenues from transactions with related party	20	$ 34	72	$ 105
Retained earnings	$ 44,017		$ 44,017		$ 41,089	$ 40,911
Lease renewal term	5 years		5 years
Retail Long-Term Care Segment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of stores | building	9,900		9,900
Number of walk in medical clinics | clinic	1,100		1,100
Health Care Benefits Segment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of people served through health insurance products and related services | person	38		38
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings					178
Retained earnings, pre-tax					$ 241
Equity Method Investee
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of states in which entity operates | state	4		4