Pension and Other Postretirement Benefit Plans Assumed Health Care Cost Trend Rates and Impact of a One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Details) (Public Service Company of New Mexico [Member], Other Postretirement Benefits [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Public Service Company of New Mexico [Member] \| Other Postretirement Benefits [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
Health care cost trend rate assumed for next year (as a percent)	7.50%	8.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (as a percent)	5.00%	5.00%
Year that the rate reaches the ultimate trend rate	2017	2017
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	$ 424
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(365)
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	6,215
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	$ (5,417)

Pension and Other Postretirement Benefit Plans Assumed Health Care Cost Trend Rates and Impact of a One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Details) (Public Service Company of New Mexico [Member], Other Postretirement Benefits [Member], USD $)
In Thousands, unless otherwise specified

12 Months Ended

Dec. 31, 2011

Dec. 31, 2010

Public Service Company of New Mexico [Member] | Other Postretirement Benefits [Member]

Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]

Health care cost trend rate assumed for next year (as a percent)

7.50%

8.00%

Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (as a percent)

5.00%

Year that the rate reaches the ultimate trend rate

2017

Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]

Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components

$ 424

Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components

(365)

Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation

6,215

Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation

$ (5,417)

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- Definition

The effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph m

-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 FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (m)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920

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- Definition

The effect of a one-percentage- point decrease in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs.

+ References

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- Definition

The effect of a one-percentage-point increase in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits.

+ References

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The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs.

+ References

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- Definition

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph l

-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 FASB

-Name Accounting Standards Codification

-Glossary Health Care Cost Trend Rate

-URI http://asc.fasb.org/extlink&oid=6514820

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

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The ultimate trend rate for health care costs.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph l

-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 FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

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The year when the ultimate health care cost trend rate is expected to be reached.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph l

-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.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

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