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Medical Claims Liability - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Insurance [Abstract] |  |  |
Amounts recorded as an adjustment to premium revenues related to minimum HBR and return of premium programs | $ 25 | $ 1 | $ 39

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- Definition Amounts recorded as an adjustment to premium revenues related to minimum HBR and return of premium programs + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration