Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 50,328		$ 45,344		$ 41,035	$ 13,711
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	[1]	us-gaap:AccountingStandardsUpdate201602Member		us-gaap:AccountingStandardsUpdate201712Member
Amortization of deferred policy acquisition costs	$ 502		$ 483		$ 406
Pharmacy Benefits Management Services | Performance Guarantee
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Performance guarantee liability	1,100		1,000
Administrative Services Only Health Care Services | Performance Guarantee
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provision for loss contingency
Amounts paid for loss contigency
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 28,575		20,162		15,088	15,800
Adjustment upon Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity			(30)	[1]	(15)	58
Adjustment upon Adoption | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity			$ (30)	[1]	$ (15)	$ 68

[1] See Note 3 for further information about the Company's adoption of new credit loss guidance (ASU 2016-13).