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Summary of the Business and Significant Accounting Policies - Inventories/Depreciation and Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Public Utilities, Inventory [Line Items]
Inventory	$ 66,417	$ 77,929
Coal
Public Utilities, Inventory [Line Items]
Inventory	12,012	24,914
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventory	54,405	53,015
Public Service Company of New Mexico
Public Utilities, Inventory [Line Items]
Inventory	60,472	72,225
Public Service Company of New Mexico | Coal
Public Utilities, Inventory [Line Items]
Inventory	12,012	24,914
Public Service Company of New Mexico | Materials and supplies
Public Utilities, Inventory [Line Items]
Inventory	$ 48,460	$ 47,311
Public Service Company of New Mexico | Electric plant
Public Utilities, Inventory [Line Items]
Depreciation average rates used	2.47%	2.47%	2.40%
Public Service Company of New Mexico | Common, intangible, and general plant
Public Utilities, Inventory [Line Items]
Depreciation average rates used	7.65%	7.91%	8.18%
Texas-New Mexico Power Company
Public Utilities, Inventory [Line Items]
Depreciation average rates used	3.95%	4.04%	3.49%
Inventory	$ 5,945	$ 5,704
Texas-New Mexico Power Company | Coal
Public Utilities, Inventory [Line Items]
Inventory	0	0
Texas-New Mexico Power Company | Materials and supplies
Public Utilities, Inventory [Line Items]
Inventory	$ 5,945	$ 5,704

X

- Definition

Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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- Definition

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+ References

No definition available.

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- Definition

Composite depreciation rate for public utility plants in service.

+ References

No definition available.

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