Retirement Benefits - Components of OCI and Changes in Regulatory Assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
Reclassification adjustments
Amortization of prior service costs	$ 30,000	$ 32,000	$ 33,000
Net periodic benefit cost	135,000	19,000	54,000
Pension Plans, Defined Benefit [Member] | Accumulated Other Comprehensive Income [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	109,000	68,000
Net (gain) loss	21,000	43,000
Change in prior service costs	0	0
Reclassification adjustments
Amortization of prior service costs	(1,000)	(1,000)
Amortization of net gain (loss)	(4,000)	(1,000)
Total reclassification adjustments	5,000	2,000
Net periodic benefit cost	16,000	41,000
Ending Balance	125,000	109,000
Pension Plans, Defined Benefit [Member] | Regulatory Assets [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	2,614,000	1,749,000
Net (gain) loss	519,000	915,000
Change in prior service costs	0	(1,000)
Reclassification adjustments
Amortization of prior service costs	(29,000)	(31,000)
Amortization of net gain (loss)	(91,000)	(20,000)
Total reclassification adjustments	120,000	51,000
Net periodic benefit cost	399,000	865,000
Ending Balance	3,013,000	2,614,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Reclassification adjustments
Amortization of prior service costs	20,000	20,000	20,000
Net periodic benefit cost	66,000	69,000	82,000
Other Postretirement Benefit Plans, Defined Benefit [Member] | Accumulated Other Comprehensive Income [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	6,000	3,000
Net (gain) loss	1,000	3,000
Change in prior service costs	0	0
Amortization of transition obligation	0	0
Reclassification adjustments
Amortization of prior service costs	0	0
Amortization of net gain (loss)	0	0
Total reclassification adjustments	0	0
Net periodic benefit cost	1,000	3,000
Ending Balance	7,000	6,000
Other Postretirement Benefit Plans, Defined Benefit [Member] | Regulatory Assets [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	345,000	292,000
Net (gain) loss	35,000	84,000
Change in prior service costs	0	12,000
Amortization of transition obligation	(10,000)	(10,000)
Reclassification adjustments
Amortization of prior service costs	(4,000)	(5,000)
Amortization of net gain (loss)	(6,000)	(4,000)
Total reclassification adjustments	20,000	19,000
Net periodic benefit cost	15,000	53,000
Ending Balance	360,000	345,000
Gulf Power [Member]
Reclassification adjustments
Amortization of prior service costs	1,262	1,262	1,302
Net periodic benefit cost	5,543	47	2,163
Gulf Power [Member] | Pension Plans, Defined Benefit [Member] | Regulatory Assets [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	115,853	75,096
Net (gain) loss	28,157	42,531
Change in prior service costs	(426)	0
Reclassification adjustments
Amortization of prior service costs	(1,262)	(1,262)
Amortization of net gain (loss)	(3,913)	(512)
Total reclassification adjustments	5,175	1,774
Net periodic benefit cost	23,408	40,757
Ending Balance	139,261	115,853
Gulf Power [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Reclassification adjustments
Amortization of prior service costs	379	396	406
Net periodic benefit cost	3,602	3,741	4,350
Gulf Power [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Regulatory Assets [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	239	0
Net (gain) loss	2,309	635
Change in prior service costs	0	0
Amortization of transition obligation	(193)	(257)
Reclassification adjustments
Amortization of prior service costs	(186)	(186)
Amortization of net gain (loss)	0	47
Total reclassification adjustments	379	396
Net periodic benefit cost	1,930	239
Ending Balance	2,169	239
Mississippi Power [Member]
Reclassification adjustments
Amortization of prior service costs	1,309	1,309	1,391
Net periodic benefit cost	8,723	3,922	6,790
Mississippi Power [Member] | Pension Plans, Defined Benefit [Member] | Regulatory Assets [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	117,354	78,130
Net (gain) loss	34,893	41,647
Change in prior service costs	0	0
Reclassification adjustments
Amortization of prior service costs	(1,309)	(1,309)
Amortization of net gain (loss)	(4,100)	(1,114)
Total reclassification adjustments	5,409	2,423
Net periodic benefit cost	29,484	39,224
Ending Balance	146,838	117,354
Mississippi Power [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Reclassification adjustments
Amortization of prior service costs	470	274	574
Net periodic benefit cost	4,111	3,815	4,816
Mississippi Power [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Regulatory Assets [Member]
Changes in the Balance of AOCI and Regulatory Assets [Roll Forward]
Beginning Balance	13,324	8,618
Net (gain) loss	2,600	4,980
Change in prior service costs	0	0
Amortization of transition obligation	(171)	(228)
Reclassification adjustments
Amortization of prior service costs	188	188
Amortization of net gain (loss)	(487)	(234)
Total reclassification adjustments	470	274
Net periodic benefit cost	2,130	4,706
Ending Balance	$ 15,454	$ 13,324