Derivatives - Foreign Currency Derivatives (Details) - 9 months ended Sep. 30, 2016 - Cash Flow Hedges Of Existing Debt [Member] - Foreign Exchange Contract [Member] € in Millions, $ in Millions	USD ($)	EUR (€)
Derivative [Line Items]
Pay Notional	$ 1,241
Receive Notional | €		€ 1,100
Fair Value Gain (Loss) at September 30, 2016	(1)
Southern Power [Member] | Maturity Date June 2022 [Member]
Derivative [Line Items]
Pay Notional	$ 677
Pay Rate	2.95%
Receive Notional | €		600
Receive Rate	1.00%
Fair Value Gain (Loss) at September 30, 2016	$ (2)
Southern Power [Member] | Maturity Date June 2026 [Member]
Derivative [Line Items]
Pay Notional	$ 564
Pay Rate	3.78%
Receive Notional | €		€ 500
Receive Rate	1.85%
Fair Value Gain (Loss) at September 30, 2016	$ 1