Financing - Short-term Debt and Liquidity (Details)		3 Months Ended	12 Months Ended
	Jul. 22, 2019 USD ($)	Mar. 31, 2021 USD ($) extension_option	Dec. 31, 2017 USD ($) derivative	Apr. 23, 2021 USD ($)	Mar. 23, 2021	Mar. 09, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 22, 2020 USD ($)	Jul. 21, 2019 USD ($)
Short-term Debt [Line Items]
Financing capacity	$ 50,000,000.0								$ 40,000,000.0
Advanced notice period prior to increasing capacity of the facility	15 days
Short-term debt		$ 197,500,000					$ 32,000,000
Letters of credit outstanding		3,400,000
Notes payable, related parties		32,400,000					0
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives			4 years
Aggregate principal amount			$ 50,000,000.0
PNMR 2020 Delayed-Draw Term Loan
Short-term Debt [Line Items]
Aggregate principal amount								$ 300,000,000.0
Long-term debt		$ 300,000,000.0
TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end		0.86%
PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt		$ 114,400,000					12,000,000
PNMR Revolving Credit Facility | PNMR
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end		1.61%
PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt		$ 40,000,000					10,000,000
Line of credit facility, interest rate at period end		1.11%
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		40,000,000.0
PNM | PNM 2019 40 Million Term Loan
Short-term Debt [Line Items]
Aggregate principal amount		40,000,000.0
PNM | PNM 2019 40 Million Term Loan | Unsecured Debt
Short-term Debt [Line Items]
Long-term debt		40,000,000.0
PNM | Senior Unsecured Notes, PCRB due 2033 at 1.875% | Senior Unsecured Notes, Pollution Control Revenue Bonds
Short-term Debt [Line Items]
Unsecured long-term debt, noncurrent		146,000,000.0
PNM | Senior Unsecured Notes, due 2021, at 5.35% | Unsecured Debt
Short-term Debt [Line Items]
Unsecured long-term debt, noncurrent		160,000,000.0
PNM | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt		0					0
PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Short-term debt		0					10,000,000
PNM | PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Letters of credit outstanding		0
TNMP
Short-term Debt [Line Items]
Short-term debt		43,100,000					0
Letters of credit outstanding		0
Notes payable, related parties		0					0
TNMP | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt		43,100,000					0
PNMR Development
Short-term Debt [Line Items]
Notes payable, related parties		0					300,000
PNMR | PNM 2019 Term Loan
Short-term Debt [Line Items]
Long-term debt		150,000,000.0
PNMR | PNMR 2020 Term Loan
Short-term Debt [Line Items]
Long-term debt		150,000,000.0
PNMR | PNMR 2020 Delayed-Draw Term Loan
Short-term Debt [Line Items]
Remaining borrowing capacity						$ 220,000,000.0
PNMR | PNMR Development Term Loan
Short-term Debt [Line Items]
Long-term debt		65,000,000.0
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity		300,000,000.0
Revolving credit facility | PNM
Short-term Debt [Line Items]
Financing capacity		400,000,000.0
Revolving credit facility | TNMP
Short-term Debt [Line Items]
Financing capacity		75,000,000.0
Revolving credit facility | TNMP | First mortgage bonds
Short-term Debt [Line Items]
Collateral amount		$ 75,000,000.0
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%
Interest rate 3 | Level 2 | Cash Flow Hedge
Short-term Debt [Line Items]
Current derivative liability		$ 300,000					$ 900,000
Subsequent event
Short-term Debt [Line Items]
Notes payable, related parties				$ 32,400,000
Remaining borrowing capacity				644,800,000
Subsequent event | PNMR
Short-term Debt [Line Items]
Remaining borrowing capacity				184,400,000
Consolidated invested cash				900,000
Subsequent event | Pollution Control Revenue Bonds | Bonds
Short-term Debt [Line Items]
Aggregate principal amount				100,300,000
Subsequent event | PNM
Short-term Debt [Line Items]
Notes payable, related parties				0
Remaining borrowing capacity				395,900,000
Consolidated invested cash				0
Subsequent event | PNM | Lines of credit
Short-term Debt [Line Items]
Remaining borrowing capacity				40,000,000.0
Subsequent event | TNMP
Short-term Debt [Line Items]
Notes payable, related parties				0
Remaining borrowing capacity				24,500,000
Consolidated invested cash				0
Subsequent event | PNMR Development
Short-term Debt [Line Items]
Notes payable, related parties				0
Remaining borrowing capacity				$ 0