Financing - Schedule Of Credit Arrangements With Project Credit Facilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Aug. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 6,505
Line of Credit Facility, Remaining Borrowing Capacity	6,428
Southern Power [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	600	$ 600	$ 500
Line of Credit Facility, Remaining Borrowing Capacity	566		$ 488
Southern Power [Member] | Construction Loan Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	235
Southern Power [Member] | Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	660
Southern Power [Member] | Construction Loan And Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	895
Line of Credit Facility, Remaining Borrowing Capacity	758
Southern Power [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	149
Line of Credit Facility, Remaining Borrowing Capacity	81
Southern Power [Member] | RE Tranquility Holdings, LLC [Member] | Construction Loan Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	86
Southern Power [Member] | RE Tranquility Holdings, LLC [Member] | Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	172
Southern Power [Member] | RE Tranquility Holdings, LLC [Member] | Construction Loan And Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	258
Line of Credit Facility, Remaining Borrowing Capacity	147
Southern Power [Member] | RE Tranquility Holdings, LLC [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	77
Line of Credit Facility, Remaining Borrowing Capacity	26
Southern Power [Member] | RE Roserock Holdings, LLC [Member] | Construction Loan Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	63
Southern Power [Member] | RE Roserock Holdings, LLC [Member] | Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	180
Southern Power [Member] | RE Roserock Holdings, LLC [Member] | Construction Loan And Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	243
Line of Credit Facility, Remaining Borrowing Capacity	243
Southern Power [Member] | RE Roserock Holdings, LLC [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	23
Line of Credit Facility, Remaining Borrowing Capacity	23
Southern Power [Member] | RE Garland Holdings, LLC [Member] | Construction Loan Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	86
Southern Power [Member] | RE Garland Holdings, LLC [Member] | Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	308
Southern Power [Member] | RE Garland Holdings, LLC [Member] | Construction Loan And Bridge Loan [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	394
Line of Credit Facility, Remaining Borrowing Capacity	368
Southern Power [Member] | RE Garland Holdings, LLC [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	49
Line of Credit Facility, Remaining Borrowing Capacity	$ 32