Contingencies and Regulatory Matters - Textual - SPC (Details) - RE Roserock, LLC - SOUTHERN POWER CO - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2015
Loss Contingencies [Line Items]
Percentage of voting interests acquired		51.00%
Withholding of construction contract payments	$ 26	$ 26

Percentage of voting equity interests acquired at the acquisition date in the business combination.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

Name:	us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant