COMMITMENTS AND CONTINGENCIES, Litigation Contingencies and Other Commitments (Details) (USD $)	3 Months Ended
	Mar. 31, 2012 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	$ 199,200,000
NPC [Member]
Royalty claim [Abstract]
Damages sought in DC Lawsuit	600,000,000
Minimum punitive damages sought in DC Lawsuit	1,000,000,000
% Owned (in hundredths)	11.00%
NPC [Member] | Capital Projects [Member]
ON Line TUA [Abstract]
Capitalized construction costs	184,000,000
SPPC [Member]
Farad Dam [Abstract]
Number of hydro generating units sold	four
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 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	$ 15,200,000