Revenue Recognition - Performance Obligations Satisfied or Partially Satisfied in Prior Periods (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2019 |
Sep. 30, 2018 |
Sep. 30, 2019 |
Sep. 30, 2018 |
|
| Revenue Recognition | ||||
| Impact to revenue from performance obligations satisfied (or partially satisfied) in previous periods | $ (42) | $ (25) | $ (33) | $ (50) |