Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Jun. 30, 2019 |
|
| Fair Value of Financial Instruments [Line Items] | ||
| Investment excludes receivables, payables and accrued income | $ 4 | $ (15) |
| Cash balance excluded from available for sale securities | 20 | (2) |
| Investments not required to be disclosed | $ 253 | $ 275 |
| NUG contracts | ||
| Fair Value of Financial Instruments [Line Items] | ||
| Period of future observable data to determine contract price | 2 years |