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Summary of the Business and Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) utility	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies Disclosures [Line Items]
Payment defaults	$ 0
Allowance for equity funds used during construction	$ 11,254,000	$ 9,478,000	$ 10,404,000
PNMR
Accounting Policies Disclosures [Line Items]
Number of regulated utilities | utility	2
Restricted cash	$ 0	0
Public Service Company of New Mexico
Accounting Policies Disclosures [Line Items]
Restricted cash	0	0
Allowance for funds used during construction, capitalized interest	$ 3,000,000.0	$ 5,000,000.0	$ 6,100,000
Allowance for funds used during construction, capitalized interest rate	2.40%	2.99%	3.19%
Allowance for equity funds used during construction	$ 6,958,000	$ 6,656,000	$ 8,173,000
Allowance for funds used during construction, capitalized cost of equity rate	3.42%	3.95%	4.25%
Impairment losses on securities held in the NDT	$ 3,200,000	$ 5,700,000	$ 13,700,000
Texas-New Mexico Power Company
Accounting Policies Disclosures [Line Items]
Restricted cash	0	0
Allowance for funds used during construction, capitalized interest	$ 2,100,000	$ 2,400,000	$ 2,300,000
Allowance for funds used during construction, capitalized interest rate	2.20%	3.23%	3.32%
Allowance for equity funds used during construction	$ 4,300,000	$ 2,800,000	$ 2,200,000
Allowance for funds used during construction, capitalized cost of equity rate	4.42%	3.78%	3.29%

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

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