Regulatory Assets and Liabilities - Additional Information (Details) $ in Millions |
Aug. 31, 2018
USD ($)
|
Dec. 31, 2019
USD ($)
state
|
Dec. 31, 2018
USD ($)
state
|
|---|---|---|---|
| Regulatory Asset And Liabilities [Line Items] | |||
| Number of proceedings | 14 | ||
| Number of proceedings, number of states effected | 11 | ||
| Number of proceedings, number of states effected, offset additional capital investment | 1 | ||
| Number of proceedings, number of states effected, reduction of certain regulatory assets | 1 | ||
| Number of proceedings, pending | 2 | ||
| Number of proceedings, amortizing EADIT and crediting customers | 6 | ||
| Number of proceedings, pending amortization of EADIT | 8 | ||
| Deferred pension expense | |||
| Regulatory Asset And Liabilities [Line Items] | |||
| Regulatory assets underfunded status | $ | $ 375 | $ 352 | |
| Postretirement Benefit Plan Assets | |||
| Regulatory Asset And Liabilities [Line Items] | |||
| Reduction to net accrued postretirement benefit obligation | $ | $ 227 |