DERIVATIVES AND HEDGING ACTIVITIES, Derivative Instruments and Option Premiums (Details) (USD $) In Millions, unless otherwise specified		12 Months Ended
	Dec. 31, 2011 NPC [Member]	Dec. 31, 2009 NPC [Member] Swap [Member]	Dec. 31, 2011 NVE [Member]	Dec. 31, 2011 NVE [Member] Swap [Member]	Dec. 31, 2011 SPPC [Member]
Derivative [Line Items]
Underlying risk		interest rate		interest rate
Derivative, Notional Amount		$ 350
Amount of hedged item		350
Description of hedged item		$350 million 8.25% General and Refunding Mortgage Notes, Series A, due June 1, 2011	3-year Term Loan
Fixed interest rate (in hundredths)				2.81%
Option Premiums [Abstract]
Total Liabilities	$ (1.3)		$ (1.3)		$ 0