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Financing - Short-term Debt and Liquidity (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) numberOfAMIMeters	Feb. 19, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 22, 2019 USD ($)
Short-term Debt [Line Items]
Financing capacity	$ 40,000,000.0				$ 50,000,000.0
Short-term debt	32,000,000			$ 185,100,000
Letters of credit outstanding	4,700,000
JPMorgan Chase Bank, N.A.
Short-term Debt [Line Items]
Letters of credit outstanding	30,300,000
PNM Resources
Short-term Debt [Line Items]
Short-term debt	12,000,000			112,100,000
Pollution Control Revenue Bonds | Bonds
Short-term Debt [Line Items]
Aggregate principal amount			$ 100,300,000
PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity	300,000,000.0
Short-term debt	$ 12,000,000			112,100,000
PNMR Revolving Credit Facility | PNM Resources
Short-term Debt [Line Items]
Interest rates on outstanding borrowings	1.66%
PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt	$ 10,000,000			0
PNM
Short-term Debt [Line Items]
Short-term debt	10,000,000			58,000,000
Letters of credit outstanding	2,200,000
PNM | Lines of credit
Short-term Debt [Line Items]
NMPRC approved credit facility	40,000,000.0
PNM | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity	400,000,000.0
Short-term debt	0			48,000,000
PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Short-term debt	$ 10,000,000			10,000,000
Interest rates on outstanding borrowings	1.41%
TNMP
Short-term Debt [Line Items]
Short-term debt	$ 0			15,000,000
Letters of credit outstanding	0
TNMP | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity	75,000,000.0
Short-term debt	$ 0			$ 15,000,000
Interest rates on outstanding borrowings	1.15%
TNMP | TNMP Revolving Credit Facility | First Mortgage Bonds at 9.50% due 2019
Short-term Debt [Line Items]
Aggregate principal amount of bonds	$ 75,000,000.0
PNM and TNMP | PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Letters of credit outstanding	$ 0
Revolving Credit Facility | TNMP | First Mortgage Bonds at 9.50% due 2019
Short-term Debt [Line Items]
Number of extension options | numberOfAMIMeters	2
Extension option period	1 year
Subsequent Event | PNM Resources
Short-term Debt [Line Items]
Consolidated invested cash		$ 900,000
Subsequent Event | PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity		257,700,000
Subsequent Event | Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity		773,100,000
Subsequent Event | PNM
Short-term Debt [Line Items]
Consolidated invested cash		32,600,000
Subsequent Event | PNM | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity		397,800,000
Subsequent Event | PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity		40,000,000.0
Subsequent Event | TNMP
Short-term Debt [Line Items]
Consolidated invested cash		0
Subsequent Event | TNMP | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity		47,600,000
Subsequent Event | PNMR Development | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity		$ 30,000,000.0

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