Acquisitions - Narrative (Details)		9 Months Ended
	Aug. 23, 2015 USD ($) $ / shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)
Business Acquisition [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 6,520,000,000
Scenario, Plan [Member]
Business Acquisition [Line Items]
Expected Equity Issuance		1,000,000,000
Scenario, Plan [Member] | Bridge Agreement [Member]
Business Acquisition [Line Items]
Business Acquisition, Share Price | $ / shares	$ 66
Approx. Purchase Price	$ 8,200,000,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	8,000,000,000
Business Combination, Acquisition Related Costs	$ 200,000,000
Expected Debt Issuance		7,000,000,000
Expected Equity Issuance		2,000,000,000
Line of Credit Facility, Maximum Borrowing Capacity		8,100,000,000.0
AGL Resources [Member] | Scenario, Plan [Member] | Bridge Agreement [Member]
Business Acquisition [Line Items]
Business Exit Costs		201,000,000
Early Termination Reimbursable Expenses		5,000,000
Southern Power [Member]
Business Acquisition [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 600,000,000
Number of Businesses Acquired		5
Total Cost		$ 299,000,000
Southern Power [Member] | SG2 Holdings, LLC [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment			$ 707,000,000
Prepaid Long Term Service Agreements			$ 20,000,000