Acquisitions and Dispositions - Held for Sale (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets Held for Sale:
Current assets	$ 407	$ 12
Liabilities Held for Sale:
Current liabilities	355	0
Other non-current liabilities	2,835	0
SOUTHERN POWER CO
Assets Held for Sale:
Current assets	18	1
Liabilities Held for Sale:
Current liabilities	4	$ 0
Disposal Group, Held-for-sale, Not Discontinued Operations
Assets Held for Sale:
Current assets	407
Total property, plant, and equipment	4,093
Other non-current assets	574
Total Assets Held for Sale	5,074
Liabilities Held for Sale:
Current liabilities	355
Long-term debt	1,285
Accumulated deferred income taxes	542
Other non-current liabilities	1,008
Total Liabilities Held for Sale	3,190
Disposal Group, Held-for-sale, Not Discontinued Operations | SOUTHERN POWER CO
Assets Held for Sale:
Current assets	18
Total property, plant, and equipment	168
Other non-current assets	17
Total Assets Held for Sale	203
Liabilities Held for Sale:
Current liabilities	4
Long-term debt	0
Accumulated deferred income taxes	0
Other non-current liabilities	0
Total Liabilities Held for Sale	$ 4