Medical Claims Liability - Schedule of change In medical claims liability (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance, January 1	$ 4,286	$ 3,929	$ 2,298
Less: reinsurance recoverable	18	5	0
Balance, January 1, net	4,268	3,924	2,298
Acquisitions	1,204	0	1,482
Less: acquired reinsurance recoverable	8	0	0
Incurred related to:
Current year	46,484	38,225	30,946
Prior years	(427)	(374)	(310)
Total incurred	46,057	37,851	30,636
Paid related to:
Current year	41,161	34,196	28,532
Prior years	3,556	3,311	1,960
Total paid	44,717	37,507	30,492
Balance at December 31, net	6,804	4,268	3,924
Plus: reinsurance recoverable	27	18	5
Balance, December 31	$ 6,831	$ 4,286	$ 3,929

Business Acquisition, Reinsurance Recoverable For Unpaid Claims And Claims Adjustment

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The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

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Amount of increase (decrease) in the cost of settling unpaid claims after deduction of reinsurance recoveries related to the acquisition of a business. Includes, but is not limited to, claims which have been incurred but not reported.

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Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs.

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Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

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Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

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Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

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Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

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