Financing - Short-term Debt and Liquidity (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2017 USD ($) derivative	Jul. 24, 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	$ 252,549,000			$ 185,100,000
Letters of credit outstanding	4,600,000
Senior Unsecured Notes
Short-term Debt [Line Items]
Aggregate principal amount	300,000,000.0
Bonds
Short-term Debt [Line Items]
Aggregate principal amount	302,500,000
Pollution Control Revenue Bonds | Bonds
Short-term Debt [Line Items]
Aggregate principal amount	100,300,000
Long-term debt				100,300,000
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives		4 years
Aggregate principal amount		$ 50,000,000.0
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
Long-term debt	$ 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	1.68%
PNM
Short-term Debt [Line Items]
Short-term debt	$ 87,055,000			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	40,000,000			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 | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	1.43%
PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	1.44%
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	13,094,000			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
TNMP | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	0.91%
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity	$ 300,000,000.0
Short-term debt	152,400,000			112,100,000
Revolving credit facility | PNM
Short-term Debt [Line Items]
Financing capacity	400,000,000.0
Short-term debt	47,055,000			48,000,000
Revolving credit facility | TNMP
Short-term Debt [Line Items]
Financing capacity	75,000,000.0
Short-term debt	12,600,000			15,000,000
Revolving credit facility | TNMP | First mortgage bonds
Short-term Debt [Line Items]
Collateral amount	75,000,000.0
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	$ 2,100,000			$ 400,000
Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity			$ 609,300,000
Subsequent event | PNMR
Short-term Debt [Line Items]
Remaining borrowing capacity			153,500,000
Consolidated invested cash			900,000
Subsequent event | PNM
Short-term Debt [Line Items]
Remaining borrowing capacity			343,900,000
Consolidated invested cash			0
Subsequent event | PNM | Lines of credit
Short-term Debt [Line Items]
Remaining borrowing capacity			0
Subsequent event | TNMP
Short-term Debt [Line Items]
Remaining borrowing capacity			74,900,000
Consolidated invested cash			63,500,000
Subsequent event | PNMR Development
Short-term Debt [Line Items]
Remaining borrowing capacity			$ 37,000,000.0