LONG-TERM DEBT (Details) (USD $)	6 Months Ended
	Jun. 30, 2011
Scheduled maturities for long-term debt [Abstract]
2011	$ 2,188,000	[1]
2012	134,822,000
2013	395,405,000
2014	128,513,000
2015	251,039,000
Long-term Debt, maturing through 2015	911,967,000
Thereafter	4,388,183,000
Long-term Debt, gross	5,300,150,000
Unamortized Premium (Discount) Amount	(12,569,000)
Total	5,287,581,000
General and Refunding Mortgage Notes [Abstract]
Aggregate principal amount paid	350,000,000
Cash from issuance of G and R mortgage notes	248,000,000
NVE Holding Co. [Member]
Scheduled maturities for long-term debt [Abstract]
Thereafter	506,500,000
Long-term Debt, gross	506,500,000
Unamortized Premium (Discount) Amount	(1,000)
Total	506,499,000
NPC [Member]
Scheduled maturities for long-term debt [Abstract]
2011	2,188,000	[1]
2012	134,822,000
2013	145,405,000
2014	128,513,000
2015	251,039,000
Long-term Debt, maturing through 2015	661,967,000
Thereafter	2,965,266,000
Long-term Debt, gross	3,627,233,000
Unamortized Premium (Discount) Amount	(26,202,000)
Total	3,601,031,000
General and Refunding Mortgage Notes [Abstract]
Redemption date	June 1, 2011
Notional amount of interest rate swap	250,000,000
Swap Transaction Settlement Date	May 9, 2011
Settlement payment amount	14,900,000
Amortization period of regulatory asset established upon settlement of swap transaction (in years)	30 year
NPC [Member] | General and Refunding Mortgage Notes, Series Y [Member]
General and Refunding Mortgage Notes [Abstract]
Issuance date	May 12, 2011
Face amount	250,000,000
Stated interest rate (in hundredths)	5.45%
Maturity date	May 15, 2041
SPPC [Member]
Scheduled maturities for long-term debt [Abstract]
2013	250,000,000
Long-term Debt, maturing through 2015	250,000,000
Thereafter	916,417,000
Long-term Debt, gross	1,166,417,000
Unamortized Premium (Discount) Amount	13,634,000
Total	$ 1,180,051,000

[1] Amounts may differ from current portion of long-term debt as reported on the consolidated balance sheet due to the timing difference of payments and the change in obligation.