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Acquisitions (Details) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended	9 Months Ended
Acquisitions (Details) $ in Millions	May 31, 2018 facility agreement	Sep. 30, 2018 USD ($) county	Jun. 30, 2019 USD ($)	Sep. 30, 2018 county customer mi MW
Business Acquisition [Line Items]
Number of business combination agreements \| agreement	3
Number of natural gas generation facilities \| facility	2
Scenario, Forecast [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 5,100
Gulf Power Company, Florida City Gas, And Natural Gas Generation Facilities [Member] \| Scenario, Forecast [Member]
Business Acquisition [Line Items]
Business combination, consideration transferred			6,475
Cash purchase price			5,100
Assumption of debt			$ 1,400
Oleader Power Project [Member] \| Oleader Power Project [Member]
Business Acquisition [Line Items]
Natural-gas fired, simple-cycle combustion turbine electric generation facility (in megawatts) \| MW				791
Stanton Energy Center [Member] \| Scenario, Forecast [Member]
Business Acquisition [Line Items]
Business acquisition, percentage of voting interests acquired			65.00%
Stanton Energy Center [Member] \| Stanton Energy Center [Member]
Business Acquisition [Line Items]
Combined-cycle electric generation facility (in megawatts) \| MW				660
Gulf Power [Member] \| Scenario, Forecast [Member]
Business Acquisition [Line Items]
Business combination, consideration transferred			$ 5,750
Cash purchase price			4,350
Business combination, recognized identifiable assets acquired and liabilities assumed, noncurrent liabilities, long-term debt			1,400
Gulf Power [Member] \| Gulf Power [Member]
Business Acquisition [Line Items]
Number of customers acquired \| customer				460,000
Number of counties in which entity operates \| county		8		8
Number of miles of power lines \| mi				9,500
Electric generating facility capacity (in megawatts) \| MW				2,300
FCG [Member]
Business Acquisition [Line Items]
Cash purchase price		$ 530
FCG [Member] \| FCG [Member]
Business Acquisition [Line Items]
Number of customers acquired \| customer				110,000
Number of miles of natural gas pipeline \| mi				3,700
Natural Gas Generation Facilities [Member] \| Scenario, Forecast [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 195