Acquisitions - Narrative (Details) - USD ($)			3 Months Ended
	Feb. 24, 2016	Aug. 23, 2015	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Business Acquisition [Line Items]
Maximum Borrowing Capacity			$ 6,495,000,000
Construction in Progress, Gross			9,406,000,000	$ 9,082,000,000
Bridge Agreement [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs			20,000,000
Maximum Borrowing Capacity					$ 8,100,000,000
Scenario, Plan [Member] | PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	$ 431,000,000
Business Acquisition, Share Price	$ 18.75
Scenario, Plan [Member] | Bridge Agreement [Member]
Business Acquisition [Line Items]
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
Southern Power [Member]
Business Acquisition [Line Items]
Maximum Borrowing Capacity			600,000,000
Total Cost			2,200,000,000
Construction in Progress, Gross			1,535,000,000	$ 1,137,000,000
Operating Expense [Member] | Bridge Agreement [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs			6,000,000
Other Income Expense Net [Member] | Bridge Agreement [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs			$ 14,000,000