LONG-TERM DEBT, Financing Transactions (Details) (USD $)	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Financing Transactions [Abstract]
Net proceeds from issuance	$ 10,951,000	$ 0
NPC [Member]
Financing Transactions [Abstract]
Net proceeds from issuance	12,432,000	0
Amount of Terminated Facility	600,000,000
Amount drawn on credit agreement	120,000,000
Amount Of Series I Notes	130,000,000
NPC [Member] \| Secured Variable Rate Debt, Revolving Credit Facilities [Member]
Financing Transactions [Abstract]
Credit facility maximum borrowing capacity	500,000,000
Credit facility expiration date	March 2017
Credit facility interest rate description	an applicable base rate (defined as the highest of the Prime Rate, the Federal Funds Rate plus 0.5% and the LIBOR Base Rate plus 1.0%) plus a margin, or a LIBOR rate plus a margin
Base rate margin (in hundredths)	0.50%
LIBOR rate margin (in hundredths)	1.50%
Maximum percentage of total commitments in effect that limits reduction in availability (in hundredths)	50.00%
Covenant terms	maintain a ratio of consolidated indebtedness to consolidated capital, determined as of the last day of each fiscal quarter, not to exceed 0.68 to 1.00
SPPC [Member]
Financing Transactions [Abstract]
Net proceeds from issuance	(1,441,000)	0
Amount of Terminated Facility	250,000,000
SPPC [Member] \| Secured Variable Rate Debt, Revolving Credit Facilities [Member]
Financing Transactions [Abstract]
Credit facility maximum borrowing capacity	$ 250,000,000
Credit facility expiration date	March 2017
Credit facility interest rate description	an applicable base rate (defined as the highest of the Prime Rate, the Federal Funds Rate plus 0.5% and the LIBOR Base Rate plus 1.0%) plus a margin, or a LIBOR rate plus a margin
Base rate margin (in hundredths)	0.50%
LIBOR rate margin (in hundredths)	1.50%
Maximum percentage of total commitments in effect that limits reduction in availability (in hundredths)	50.00%
Covenant terms	a ratio of consolidated indebtedness to consolidated capital, determined as of the last day of each fiscal quarter, not to exceed 0.68 to 1.00

Amount of NPC's General and Refunding Mortgage Notes, Series I Notes that matured in April 2012.

No definition available.

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- Definition

Amount of credit facility terminated in April 2012.

+ References

No definition available.

+ Details

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Namespace Prefix:	nve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The applicable base rate margin used to determine the rate for outstanding loans under the credit facility.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Amount drawn on credit agreement to pay for NPC's maturing Series I notes.

+ References

No definition available.

+ Details

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Namespace Prefix:	nve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The applicable LIBOR rate margin used to determine the rate for outstanding loans under the credit facility.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

The maximum percentage of total commitments in effect that limits reduction in availability under the credit facility.

+ References

No definition available.

+ Details

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- Details

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- Definition

Description of the conditions for borrowing under the credit facility including the nature of any restrictions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Reflects when the credit facility terminates, which may be presented in a variety of ways (year, month and year, day, month and year, quarter).

+ References

+ Details

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- Definition

Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions).

+ References

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 18

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 19

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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