Financing - Short-term Debt and Liquidity (Details)		9 Months Ended	12 Months Ended
	Oct. 20, 2020 USD ($)	Sep. 30, 2020 USD ($) numberOfAMIMeters	Dec. 31, 2017 USD ($) derivative	Oct. 27, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 22, 2019 USD ($)	Jul. 21, 2019 USD ($)
Short-term Debt [Line Items]
Financing capacity						$ 50,000,000.0	$ 40,000,000.0
Short-term debt		$ 184,294,000			$ 185,100,000
Letters of credit outstanding		4,700,000
Bonds
Short-term Debt [Line Items]
Aggregate principal amount		302,500,000
Senior Unsecured Notes
Short-term Debt [Line Items]
Aggregate principal amount		$ 300,000,000.0
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives			4 years
Aggregate principal amount			$ 50,000,000.0
Pollution Control Revenue Bonds | Bonds
Short-term Debt [Line Items]
Stated interest rate		0.32%
PNMR Development Term Loan
Short-term Debt [Line Items]
Aggregate principal amount		$ 90,000,000.0
PNMR 2018 Two Year Term Loan
Short-term Debt [Line Items]
Aggregate principal amount		$ 50,000,000.0
Term of loan		2 years
PNMR Revolving Credit Facility | PNMR
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end		1.66%
PNM
Short-term Debt [Line Items]
Short-term debt		$ 21,700,000			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
Short-term debt		20,000,000			10,000,000
PNM | PNM 2019 Term Loan | Term loan agreement with banks
Short-term Debt [Line Items]
Aggregate principal amount		150,000,000.0
PNM | PNM 2019 40 Million Term Loan
Short-term Debt [Line Items]
Aggregate principal amount		40,000,000.0
PNM | PNM 2020 250 Million Term Loan | Term loan agreement with banks
Short-term Debt [Line Items]
Aggregate principal amount		150,000,000.0
PNM | 5.35% Senior Unsecured Notes due October 2021
Short-term Debt [Line Items]
Aggregate principal amount		$ 160,000,000.0
Stated interest rate		5.35%
PNM | Pollution Control Revenue Bonds at 1.875% | Bonds
Short-term Debt [Line Items]
Aggregate principal amount		$ 146,000,000.0
Stated interest rate		1.875%
PNM | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end		1.40%
PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end		1.41%
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		494,000			15,000,000
Letters of credit outstanding		100,000
TNMP | PNMR Development Term Loan | Term loan agreement with banks
Short-term Debt [Line Items]
Aggregate principal amount		90,000,000.0
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity		300,000,000.0
Short-term debt		162,100,000			112,100,000
Revolving credit facility | PNM
Short-term Debt [Line Items]
Financing capacity		400,000,000.0
Short-term debt		1,700,000			48,000,000
Revolving credit facility | TNMP
Short-term Debt [Line Items]
Financing capacity		75,000,000.0
Short-term debt		0			15,000,000
Revolving credit facility | TNMP | First mortgage bonds
Short-term Debt [Line Items]
Collateral amount		$ 75,000,000.0
Number of extension options | numberOfAMIMeters		2
Extension option period		1 year
Bonds | TNMP
Short-term Debt [Line Items]
Short-term debt		$ 494,000			0
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%
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		$ 1,500,000			$ 400,000
Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity				$ 722,400,000
Subsequent event | PNMR
Short-term Debt [Line Items]
Remaining borrowing capacity				129,700,000
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]
Remaining borrowing capacity				397,800,000
Consolidated invested cash				13,800,000
Subsequent event | PNM | Lines of credit
Short-term Debt [Line Items]
Remaining borrowing capacity				30,000,000.0
Subsequent event | TNMP
Short-term Debt [Line Items]
Remaining borrowing capacity				74,900,000
Consolidated invested cash				22,800,000
Subsequent event | PNMR Development
Short-term Debt [Line Items]
Remaining borrowing capacity				$ 40,000,000.0
Subsequent event | Term Loan | PNMR | Merger Backstop Facilities
Short-term Debt [Line Items]
Financing capacity	$ 50,000,000.0
Remaining borrowing capacity	$ 50,000,000.0
Term of loan	364 days