Regulatory Matters - Alabama Power (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2026	Dec. 31, 2026	Dec. 31, 2025
Loss Contingencies [Line Items]
In service	$ 149,086		$ 146,114
ALABAMA POWER CO
Loss Contingencies [Line Items]
In service	40,185		38,915
ALABAMA POWER CO | Plant Barry Unit 5
Loss Contingencies [Line Items]
In service	307
SOUTHERN POWER CO
Loss Contingencies [Line Items]
In service	15,342		$ 15,034
SOUTHERN POWER CO | Plant Barry Unit 5
Loss Contingencies [Line Items]
In service	307
Reliability Reserve Liability | ALABAMA POWER CO
Loss Contingencies [Line Items]
Remaining net book value	163
Reliability reserve expense	$ 21
Reliability Reserve Liability | ALABAMA POWER CO | Scenario, Forecast
Loss Contingencies [Line Items]
Reliability reserve expense		$ 60