SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010		Sep. 30, 2011	Sep. 30, 2010
Schedule of Prior Period Adjustment [Line Items]
Operating income	$ 353,196,000	$ 343,364,000	[1]	$ 533,981,000	$ 541,000,000	[1]
Net income, net of taxes	173,462,000	177,546,000	[2]	188,680,000	212,771,000	[2]
NPC [Member]
Schedule of Prior Period Adjustment [Line Items]
Operating income	296,327,000	288,163,000	[1]	410,037,000	411,962,000
Net income, net of taxes	154,608,000	158,928,000		161,651,000	176,386,000
NPC [Member] | Terminated Contracts [Member] | Revision Adjustment [Member]
Schedule of Prior Period Adjustment [Line Items]
Operating income		(8,000,000)			(8,000,000)
Net income, net of taxes		(5,200,000)			(5,200,000)
Revision Adjustment [Member]
Schedule of Prior Period Adjustment [Line Items]
Operating income		(8,000,000)			(8,000,000)
Net income, net of taxes		$ (5,200,000)			$ (5,200,000)
Basic and diluted earnings per share, net of taxes (in dollars per share)		$ (0.02)			$ (0.02)

[1] During the third quarter 2010, NPC terminated a long-term service agreement from one of its generating stations. The estimated termination payment was not considered material to the third quarter but would have been material to the fourth quarter; therefore, as disclosed in our 2010 Form 10-K, third quarter 2010 was revised to reflect the estimated termination payment. As such, operating income for the three and nine months ended September 30, 2010 was reduced by $8.0 million, before tax, for NVE and NPC, from that reported in the Forms 10-Q for the quarterly period ended September 30, 2010. See Note 1, Summary of Significant Accounting Policies.

[2] As discussed in Note 1, Summary of Significant Accounting Policies, net income and earnings per share basic and diluted were reduced from that previously reported by $5.2 million (net of taxes) and $0.02 per share (net of taxes), respectively.