Commitments and Contingencies - PVNGS Decommissioning Funding (Details) - Public Service Company of New Mexico - Palo Verde Nuclear Generating Station - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Funding for decommissioning costs in qualified and non-qualified trust funds	$ 2.0	$ 4.2	$ 4.9
Estimated market value of trusts for decommissioning costs	$ 293.7	$ 253.9

The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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- Definition

The cash outflow for the purchase of investments that will be held in a decommissioning trust fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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