SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 number	Dec. 31, 2010	Dec. 31, 2009
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
CWIP	$ 487,427,000	$ 908,579,000
Total Utility Property, Gross	12,411,144,000	11,977,097,000
Regulated and Unregulated Operating Revenue [Abstract]
Collection period for Alternative Revenues (in months)		24 months
Asset retirement obligation [Roll Forward]
ARO balance at January 1	55,202,000	55,968,000
Liabilities incurred in current period	3,281,727.45
Liabilities settled in current period	6,996,384.55	34,000
Accretion expense	3,865,564.81	3,877,000
Revision in estimated cash flows	16,390,842.9	(4,606,000)
Gain Loss On Settlement	(762,606.45)	(3,000)
ARO balance at December 31	70,982,000	55,202,000
NPC [Member]
Basis of Presentation
Percentage of subsidiary assets to consolidated assets (in hundredths)	73.00%
Number of electricity customers	840,000
Equity Carrying Charges
Accumulated equity carrying charges	12,700,000	12,000,000
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
CWIP	352,541,000	825,079,000
Total Utility Property, Gross	8,698,312,000	8,377,176,000
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Public Utilities, Allowance for Funds Used During Construction, Rate		8.32%	8.57%
Depreciation
Composite depreciation rate for plants in service (in hundredths)	3.04%	2.99%	2.74%
Regulated and Unregulated Operating Revenue [Abstract]
Unbilled Receivables, Current	93,000,000	89,000,000
Asset retirement obligation [Roll Forward]
ARO balance at January 1	47,126,000	48,320,000
Liabilities incurred in current period	3,281,727.45
Liabilities settled in current period	6,996,384.55	34,000
Accretion expense	3,348,038.66	3,383,000
Revision in estimated cash flows	15,021,213.94	(4,540,000)
Gain Loss On Settlement	(762,606.45)	(3,000)
ARO balance at December 31	61,017,989.05	47,126,000	48,320,000
Cost of Removal
Regulatory liabilities - cost of removal	232,000,000	208,800,000
SPPC [Member]
Basis of Presentation
Percentage of subsidiary assets to consolidated assets (in hundredths)	27.00%
Number of electricity customers	323,000
Electricity service area (in square miles)	50,000
Number of natural gas customers	152,000
Equity Carrying Charges
Accumulated equity carrying charges	900,000	1,100,000
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
CWIP	134,886,000	83,500,000
Total Utility Property, Gross	3,712,832,000	3,599,921,000
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Public Utilities, Allowance for Funds Used During Construction, Rate			7.96%
Depreciation
Composite depreciation rate for plants in service (in hundredths)	2.89%	3.02%	3.07%
Regulated and Unregulated Operating Revenue [Abstract]
Unbilled Receivables, Current	51,000,000	60,000,000
Asset retirement obligation [Roll Forward]
ARO balance at January 1	8,076,000	7,648,000
Liabilities incurred in current period
Liabilities settled in current period
Accretion expense	517,526.15	494,000
Revision in estimated cash flows	1,369,628.96	(66,000)
Gain Loss On Settlement
ARO balance at December 31	9,964,286.27	8,076,000	7,648,000
Cost of Removal
Regulatory liabilities - cost of removal	189,900,000	173,500,000
General Plant [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
General, Intangible plant	659,000,000	657,000,000
CWIP	473,000,000	906,000,000
Common Assets	197,000,000	191,000,000
General Plant [Member] | NPC [Member]
Estimated useful lives by major class [Abtract]
General Plant assets (in years)	5 to 65 years
General Plant [Member] | SPPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Common Assets	197,000,000	191,000,000
Estimated useful lives by major class [Abtract]
General Plant assets (in years)	5 to 65 years
Electric General Plant [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation assets	4,791,000,000	4,056,000,000
Electric General Plant [Member] | NPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation assets	3,724,000,000	2,991,000,000
General, Intangible plant	564,000,000	558,000,000
CWIP	353,000,000	825,000,000
Estimated useful lives by major class [Abtract]
Generation assets (in years)	25 to 125 years
Electric General Plant [Member] | SPPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Generation assets	1,067,000,000	1,065,000,000
General, Intangible plant	95,000,000	99,000,000
CWIP	121,000,000	81,000,000
Estimated useful lives by major class [Abtract]
Generation assets (in years)	25 to 125 years
Transmission assets (in years)	50 to 70 years
Natural Gas General Plant [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Distribution assets	312,000,000	303,000,000
General, Intangible plant	3,000,000	3,000,000
CWIP	14,000,000	2,000,000
Natural Gas General Plant [Member] | NPC [Member]
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Public Utilities, Allowance for Funds Used During Construction, Rate	8.47%
Natural Gas General Plant [Member] | SPPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
General, Intangible plant	3,000,000	3,000,000
CWIP	14,000,000	2,000,000
Electric [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Transmission assets	1,853,000,000	1,840,000,000
Distribution assets	4,108,000,000	4,019,000,000
Electric [Member] | NPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Transmission assets	1,183,000,000	1,183,000,000
Distribution assets	2,874,000,000	2,820,000,000
Estimated useful lives by major class [Abtract]
Transmission assets (in years)	45 to 65 years
Distribution assets (in years)	20 to 65 years
Electric [Member] | SPPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Transmission assets	670,000,000	657,000,000
Distribution assets	1,234,000,000	1,199,000,000
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Public Utilities, Allowance for Funds Used During Construction, Rate	7.86%
Estimated useful lives by major class [Abtract]
Distribution assets (in years)	30 to 65 years
Natural Gas [Member] | NPC [Member]
Estimated useful lives by major class [Abtract]
Distribution assets (in years)	N/A
Natural Gas [Member] | SPPC [Member]
Public Utilities, Property, Plant and Equipment, Plant in Service [Abstract]
Distribution assets	$ 312,000,000	$ 303,000,000
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Public Utilities, Allowance for Funds Used During Construction, Rate	5.15%
Estimated useful lives by major class [Abtract]
Distribution assets (in years)	40 to 70 years