Consolidated Entities and Equity Method Investments - Balance Sheet Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions |
Jun. 30, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Schedule of Equity Method Investments [Line Items] | ||
| Investment Balance | $ 1,557 | $ 1,580 |
| SOUTHERN Co GAS | ||
| Schedule of Equity Method Investments [Line Items] | ||
| Investment Balance | 1,509 | 1,538 |
| SOUTHERN Co GAS | SNG | ||
| Schedule of Equity Method Investments [Line Items] | ||
| Investment Balance | 1,243 | 1,261 |
| SOUTHERN Co GAS | Atlantic Coast Pipeline | ||
| Schedule of Equity Method Investments [Line Items] | ||
| Investment Balance | 101 | 83 |
| SOUTHERN Co GAS | PennEast Pipeline | ||
| Schedule of Equity Method Investments [Line Items] | ||
| Investment Balance | 77 | 71 |
| SOUTHERN Co GAS | Other | ||
| Schedule of Equity Method Investments [Line Items] | ||
| Investment Balance | $ 88 | $ 123 |