Fair Value of Derivative and Other Financial Instruments, Sale of Power (Details) (Public Service Company of New Mexico [Member], USD $) In Millions, unless otherwise specified	32 Months Ended
Dec. 31, 2010 mw
Firm Contract [Member]
Sale of Power [Line Items]
Long Term Contract for Sale of Electric Power Plant Output	90,000
Firm Contract 1 [Member]
Sale of Power [Line Items]
Prepayments	40.6
Firm Contract 2 [Member]
Sale of Power [Line Items]
Prepayments	30.0
Contingent Contract [Member]
Sale of Power [Line Items]
Long Term Contract for Sale of Electric Power Plant Output	45,000

The increase (decrease) during the reporting period in the amount of prepayments by customers for goods or services to be provided at a later date.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Name:	us-gaap_IncreaseDecreaseInCustomerAdvances
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration