Financing - Schedule of Line of Credit Facilities (Details)	Sep. 30, 2016 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 8,445,000,000
Remaining borrowing capacity	8,281,000,000
Southern Power [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	600,000,000
Remaining borrowing capacity	532,000,000
Southern Power [Member] | Construction Loans [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	235,000,000
Southern Power [Member] | Construction Loans [Member] | RE Garland Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	86,000,000
Southern Power [Member] | Construction Loans [Member] | RE Roserock Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	63,000,000
Southern Power [Member] | Construction Loans [Member] | RE Tranquility Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	86,000,000
Southern Power [Member] | Bridge Loan [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	660,000,000
Southern Power [Member] | Bridge Loan [Member] | RE Garland Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	308,000,000
Southern Power [Member] | Bridge Loan [Member] | RE Roserock Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	180,000,000
Southern Power [Member] | Bridge Loan [Member] | RE Tranquility Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	172,000,000
Southern Power [Member] | Construction Loan And Bridge Loan [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	895,000,000
Remaining borrowing capacity	67,000,000
Southern Power [Member] | Construction Loan And Bridge Loan [Member] | RE Garland Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	394,000,000
Remaining borrowing capacity	21,000,000
Southern Power [Member] | Construction Loan And Bridge Loan [Member] | RE Roserock Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	243,000,000
Remaining borrowing capacity	34,000,000
Southern Power [Member] | Construction Loan And Bridge Loan [Member] | RE Tranquility Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	258,000,000
Remaining borrowing capacity	12,000,000
Southern Power [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	149,000,000
Remaining borrowing capacity	65,000,000
Southern Power [Member] | Letter of Credit [Member] | RE Garland Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	49,000,000
Remaining borrowing capacity	23,000,000
Southern Power [Member] | Letter of Credit [Member] | RE Roserock Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	23,000,000
Remaining borrowing capacity	16,000,000
Southern Power [Member] | Letter of Credit [Member] | RE Tranquility Holdings, LLC [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	77,000,000
Remaining borrowing capacity	$ 26,000,000