COMMITMENTS AND CONTINGENCIES, Litigation Contingencies and Other Commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 MW
Capital Projects [Member]
ON Line TUA [Abstract]
Capacity of interconnection (in kV)	500
Utilities' aggregate share in transmission line project (in hundredths)	25.00%
GBT-South's share in the ON Line project (in hundredths)	75.00%
Capitalized construction costs	$ 152,300,000
Total estimated construction costs	556,000,000
NPC [Member]
Royalty claim [Abstract]
Damages sought in DC Lawsuit	600,000,000
Minimum punitive damages sought in DC Lawsuit	1,000,000,000
NPC [Member] | Capital Projects [Member]
ON Line TUA [Abstract]
Capitalized construction costs	144,100,000
SPPC [Member]
Farad Dam [Abstract]
Total capacity of hydro generating units sold (in MW)	10.3
Proceeds from sale of hydro units	8,000,000
Capacity of the Farad Hydro generating unit (in MW)	2.8
Estimate to rebuild diversion dam of the Farad Hydro generating unit	20,000,000
Amount entitled to be recovered for costs incurred to date on Farad Dam	4,000,000
Duration to rebuild the Farad dam from the date of the Court's decision	three
Amount of actual cash value entitled to if Farad Dam is not rebuilt	1,300,000
SPPC [Member] | Capital Projects [Member]
ON Line TUA [Abstract]
Capitalized construction costs	$ 8,200,000