Summary of Significant Accounting Policies - Reconciliation of Cash Cash Equivalent and Restricted Cash As Presented in SCF (Details) - USD ($) $ in Millions |
Sep. 30, 2019 |
Dec. 31, 2018 |
Sep. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|---|---|
| Accounting Policies [Abstract] | ||||
| Cash and cash equivalents | $ 106 | $ 101 | ||
| Restricted cash - current | 0 | 4 | ||
| Restricted cash | 13 | 18 | ||
| Cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows | $ 119 | $ 123 | $ 146 | $ 138 |