SEGMENT INFORMATION, PRIOR PERIOD ADJUSTMENT (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]
Revenues	$ 1,017,796,000	[1]	$ 1,128,039,000	[2],[3]	$ 2,333,710,000	[1]	$ 2,625,211,000	[2],[3]
Gross Margin	601,152,000		586,074,000	[2],[3]	1,251,043,000		1,244,069,000	[2],[3]
Operating income, before tax	353,196,000		343,364,000	[3]	533,981,000		541,000,000	[3]
NPC [Member]
Schedule Of Prior Period Adjustment [Line Items]
Revenues	798,914,000	[1]	870,950,000	[2],[3]	1,662,880,000	[1]	1,836,144,000	[2],[3]
Operating income, before tax	296,327,000		288,163,000	[3]	410,037,000		411,962,000
NPC [Member] | Amounts for REPR Presented Net [Member] | Revision Adjustment [Member]
Schedule Of Prior Period Adjustment [Line Items]
Revenues			(2,000,000)				(4,600,000)
Gross Margin			(2,000,000)				(4,600,000)
NPC [Member] | Terminated Contracts [Member] | Revision Adjustment [Member]
Schedule Of Prior Period Adjustment [Line Items]
Operating income, before tax			(8,000,000)				(8,000,000)
SPPC [Member]
Schedule Of Prior Period Adjustment [Line Items]
Revenues	218,878,000	[1]	257,084,000		670,819,000	[1]	789,048,000
Operating income, before tax	57,574,000		56,223,000		126,426,000		131,811,000
SPPC [Member] | Amounts for REPR Presented Net [Member] | Revision Adjustment [Member]
Schedule Of Prior Period Adjustment [Line Items]
Revenues			(1,500,000)				(4,100,000)
Gross Margin			(1,500,000)				(4,100,000)
Amounts for REPR Presented Net [Member] | Revision Adjustment [Member]
Schedule Of Prior Period Adjustment [Line Items]
Revenues			(3,500,000)				(8,700,000)
Gross Margin			(3,500,000)				(8,700,000)
Revision Adjustment [Member]
Schedule Of Prior Period Adjustment [Line Items]
Operating income, before tax			$ (8,000,000)				$ (8,000,000)

[1] Effective July 1, 2011, included in operating revenues were EEPR revenues, for which costs related to the program are included in other operating expense and therefore have no effect on operating income. See Note 3, Regulatory Actions.

[2] As reported in our 2010 Form 10-K, amounts for REPR are presented net. As such, revenues and gross margin for the three months ended September 30, 2010 were reduced by $3.5 million, $2.0 million and $1.5 million for NVE, NPC and SPPC, respectively, from that reported in the Forms 10-Q for the period ended September 30, 2010. Revenues and gross margin for the nine months ended September 30, 2010 were reduced by $8.7 million, $4.6 million and $4.1 million for NVE, NPC and SPPC, respectively, from that reported in the Forms 10-Q for the period ended September 30, 2010.

[3] 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.