Revenue - Schedule of Disaggregation of Revenue (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Outsourced administration [Member] \| HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue prior period reclassification adjustment	$ 48	$ 106

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration