Financing - Short-term Debt and Liquidity (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) extension_option	Dec. 31, 2017 USD ($) derivative	Oct. 22, 2021 USD ($)	Mar. 23, 2021	Dec. 31, 2020 USD ($)
Short-term Debt [Line Items]
Financing capacity	$ 40,000,000
Short-term debt	13,900,000				$ 32,000,000
Letters of credit outstanding	3,400,000
Notes payable, related parties	0				0
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives		4 years
Aggregate principal amount		$ 50,000,000
PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt	$ 13,900,000				12,000,000
PNMR Revolving Credit Facility | PNMR
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	1.58%
PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt	$ 0				10,000,000
PNM
Short-term Debt [Line Items]
Short-term debt	0				10,000,000
Letters of credit outstanding	2,200,000
Notes payable, related parties	0				0
PNM | Lines of credit
Short-term Debt [Line Items]
NMPRC approved credit facility	40,000,000
PNM | Senior Unsecured Notes PCRB, Reprice Date of June 2022 | Senior Unsecured Notes, Pollution Control Revenue Bonds
Short-term Debt [Line Items]
Unsecured long-term debt, noncurrent	104,500,000
PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Short-term debt	0				10,000,000
Letters of credit outstanding	0
PNM | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Letters of credit outstanding	0
TNMP
Short-term Debt [Line Items]
Letters of credit outstanding	0
Notes payable, related parties	0				0
PNMR Development
Short-term Debt [Line Items]
Notes payable, related parties	100,000				$ 300,000
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity	300,000,000
Revolving credit facility | PNM
Short-term Debt [Line Items]
Financing capacity	400,000,000
Revolving credit facility | TNMP
Short-term Debt [Line Items]
Financing capacity	75,000,000
Revolving credit facility | TNMP | First mortgage bonds
Short-term Debt [Line Items]
Collateral amount	$ 75,000,000
Number of extension options | extension_option	2
Extension option period	1 year
Interest rate contract
Short-term Debt [Line Items]
Number of derivatives | derivative		3
Interest rate 1 | Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate		1.926%		1.926%
Interest rate 2 | Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate		1.823%
Interest rate 3 | Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate		1.629%
Subsequent event
Short-term Debt [Line Items]
Notes payable, related parties			$ 0
Remaining borrowing capacity			798,800,000
Subsequent event | PNMR
Short-term Debt [Line Items]
Remaining borrowing capacity			286,000,000
Consolidated invested cash			900,000
Subsequent event | PNM
Short-term Debt [Line Items]
Notes payable, related parties			0
Remaining borrowing capacity			397,800,000
Consolidated invested cash			61,800,000
Subsequent event | PNM | Lines of credit
Short-term Debt [Line Items]
Remaining borrowing capacity			40,000,000
Subsequent event | TNMP
Short-term Debt [Line Items]
Notes payable, related parties			0
Remaining borrowing capacity			75,000,000
Consolidated invested cash			11,400,000
Subsequent event | PNMR Development
Short-term Debt [Line Items]
Notes payable, related parties			$ 200,000