Summary of Significant Accounting Policies - Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable, Net [Abstract]
Insurance customer receivables	$ 1,888	$ 1,818
Noninsurance customer receivables	4,988	441
Pharmaceutical manufacturer receivable	3,321	645
Other receivables	276	251
Total accounts receivable, net	10,473	3,155
Pharmaceutical manufacturer receivables under noninsurance customer contracts	406	336
Unbilled receivables	1,200
Allowance for doubtful accounts for receivables	217	210
Allowances for uncollectible reinsurance
Amortization of deferred policy acquisition costs	406	322	$ 292
Performance Guarantee [Member]
Loss Contingencies [Line Items]
Reserves for performance guarantees	$ 895