Medical Claims Liability - Narrative (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Insurance [Abstract] | |||
| Amounts recorded as an adjustment to premium revenues related to minimum HBR and return of premium programs | $ 1 | $ 39 | $ 47 |