Employee Benefits - Summary of Accumulated Other Comprehensive Income and Regulatory Assets (Details) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Pension Plan Asset | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Net actuarial loss | $ 435 | $ 431 |
| Prior service credit | (19) | (22) |
| Net amount recognized | 416 | 409 |
| Regulatory assets (liabilities) | 375 | 352 |
| Accumulated other comprehensive income | 41 | 57 |
| Postretirement Benefit Plan Assets | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Net actuarial loss | 72 | 83 |
| Prior service credit | (257) | (291) |
| Net amount recognized | (185) | (208) |
| Regulatory assets (liabilities) | (185) | (208) |
| Accumulated other comprehensive income | $ 0 | $ 0 |