Financing - Financing Activities (Details)										6 Months Ended
	Jun. 22, 2020 USD ($)	Apr. 30, 2020 USD ($)	Apr. 24, 2020 USD ($) bond_series	Apr. 15, 2020 USD ($)	Apr. 01, 2020 USD ($)	Jan. 08, 2020 $ / shares shares	Jan. 07, 2020 shares	Jan. 01, 2020	Jul. 01, 2019 USD ($)	Jun. 30, 2020 USD ($) shares	Jun. 30, 2019	Jul. 24, 2020	Jul. 15, 2020 USD ($)	Jun. 01, 2020 USD ($)	Dec. 31, 2019 USD ($) shares	Oct. 21, 2016 USD ($)
Debt Instrument [Line Items]
Common stock, shares issued (in shares) | shares										79,653,624					79,653,624
PNMR 2020 Forward Equity Sales Agreement
Debt Instrument [Line Items]
Common stock, shares issued (in shares) | shares							6,200,000
Sale of stock, price per share (in dollars per share) | $ / shares						$ 47.21
Over-Allotment Option
Debt Instrument [Line Items]
Number of shares issued in transaction (in shares) | shares						800,000	800,000
PNMR 2018 Two Year Term Loan
Debt Instrument [Line Items]
Aggregate principal amount										$ 50,000,000.0
Variable interest rate										0.98%
Term loans										$ 50,000,000.0
Term of loan										2 years
PNMR 2019 Term Loan
Debt Instrument [Line Items]
Variable interest rate										1.14%
PNMR Development Term Loan
Debt Instrument [Line Items]
Aggregate principal amount										$ 90,000,000.0
PNM
Debt Instrument [Line Items]
Common stock, shares issued (in shares) | shares										39,117,799					39,117,799
PNM | PNM 2019 40 Million Term Loan
Debt Instrument [Line Items]
Aggregate principal amount										$ 40,000,000.0
Variable interest rate										0.84%
PNM | Maximum
Debt Instrument [Line Items]
Maturity term over which financings require regulator approval (more than)										18 months
Texas-New Mexico Power Company
Debt Instrument [Line Items]
Common stock, shares issued (in shares) | shares										6,358					6,358
Line of credit
Debt Instrument [Line Items]
Ratio of debt to capital (less than or equal to)										65.00%	70.00%
Line of credit | PNM | PNM Revolving Credit Facility
Debt Instrument [Line Items]
Repayments of debt		$ 100,000,000.0
Letter of credit | PNMR | JPM LOC Facility
Debt Instrument [Line Items]
Aggregate principal amount																$ 30,300,000
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement
Debt Instrument [Line Items]
Ratio of debt to capital (less than or equal to)			65.00%
Aggregate principal amount			$ 185,000,000.0
Series of bond issuance | bond_series			4
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement | Subsequent event
Debt Instrument [Line Items]
Aggregate principal amount													$ 185,000,000.0
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 1 and 2
Debt Instrument [Line Items]
Aggregate principal amount			$ 110,000,000.0
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 1
Debt Instrument [Line Items]
Aggregate principal amount			$ 85,000,000.0
Stated interest rate			2.73%
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 2
Debt Instrument [Line Items]
Aggregate principal amount			$ 25,000,000.0
Stated interest rate			3.36%
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 3 and 4 | Subsequent event
Debt Instrument [Line Items]
Aggregate principal amount													75,000,000.0
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 3 | Subsequent event
Debt Instrument [Line Items]
Aggregate principal amount													$ 25,000,000.0
Stated interest rate													2.93%
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 4 | Subsequent event
Debt Instrument [Line Items]
Aggregate principal amount													$ 50,000,000.0
Stated interest rate													3.36%
Medium-term Notes | PNM | PNM 2020 250 Million Term Loan
Debt Instrument [Line Items]
Aggregate principal amount				$ 250,000,000.0
Variable interest rate										2.70%
Medium-term Notes | PNM | PNM 2019 Term Loan
Debt Instrument [Line Items]
Repayments of debt				$ 250,000,000.0
Senior Unsecured Notes
Debt Instrument [Line Items]
Aggregate principal amount										$ 300,000,000.0
Senior Notes | Senior Unsecured Notes, 3.21% Series A, due April 2030
Debt Instrument [Line Items]
Aggregate principal amount		$ 150,000,000.0
Stated interest rate		3.21%
Senior Notes | Senior Unsecured Notes, 3.57% Series B, due April 2039
Debt Instrument [Line Items]
Aggregate principal amount		$ 50,000,000.0
Stated interest rate		3.57%
Senior Notes | PNM
Debt Instrument [Line Items]
Ratio of debt to capital (less than or equal to)		65.00%
Aggregate principal amount		$ 200,000,000.0
Senior Notes | PNM | PNM 2020 250 Million Term Loan
Debt Instrument [Line Items]
Repayments of debt		$ 100,000,000.0
Bonds
Debt Instrument [Line Items]
Aggregate principal amount										302,500,000
Repayments of debt	$ 302,500,000
Bonds | City of Farmington, New Mexico
Debt Instrument [Line Items]
Aggregate principal amount														$ 266,500,000
Bonds | Pollution Control Revenue Bonds
Debt Instrument [Line Items]
Aggregate principal amount										100,300,000
Term loans															$ 100,300,000
Bonds | Pollution Control Revenue Bonds | Subsequent event
Debt Instrument [Line Items]
Stated interest rate												0.47%
Bonds | Pollution Control Revenue Bonds, Due 2040
Debt Instrument [Line Items]
Term loans															40,000,000.0
Bonds | Pollution Control Revenue Bonds, Due 2043, Series 1
Debt Instrument [Line Items]
Term loans															39,300,000
Bonds | Pollution Control Revenue Bonds, Due 2043, Series 2
Debt Instrument [Line Items]
Term loans															21,000,000.0
Bonds | Pollution Control Revenue Bonds, 5.90%, Callable | City of Farmington, New Mexico
Debt Instrument [Line Items]
Aggregate principal amount										$ 255,000,000.0					$ 255,000,000.0
Stated interest rate										5.90%					5.90%
Bonds | Pollution Control Revenue Bonds, 6.25%, Callable | Maricopa County, Arizona Pollution Control Corporation
Debt Instrument [Line Items]
Aggregate principal amount										$ 36,000,000.0					$ 36,000,000.0
Stated interest rate															6.25%
Debt instrument, redemption price, percentage								101.00%
Bonds | Pollution Control Revenue Bonds, 6.25%, Callable | City of Farmington, New Mexico
Debt Instrument [Line Items]
Aggregate principal amount										$ 11,500,000					$ 11,500,000
Stated interest rate										6.25%					6.25%
Bonds | Pollution Control Revenue Bonds, 1.05 Percent Due 2022
Debt Instrument [Line Items]
Aggregate principal amount										$ 36,000,000.0
Stated interest rate										1.05%
Bonds | Pollution Control Revenue Bonds, 1.20 Percent Due 2022
Debt Instrument [Line Items]
Aggregate principal amount										$ 11,500,000
Stated interest rate										1.20%
Bonds | Pollution Control Revenue Bonds, 1.10 Percent Due 2023
Debt Instrument [Line Items]
Aggregate principal amount										$ 130,000,000.0
Stated interest rate										1.10%
Bonds | Pollution Control Revenue Bonds, 1.15 Percent Due 2024
Debt Instrument [Line Items]
Aggregate principal amount										$ 125,000,000.0
Stated interest rate										1.15%
Term loan agreement with banks | PNM | PNM 2020 250 Million Term Loan
Debt Instrument [Line Items]
Aggregate principal amount										$ 150,000,000.0
Term loan agreement with banks | PNM | PNM 2019 Term Loan
Debt Instrument [Line Items]
Aggregate principal amount										$ 150,000,000.0
Term loan agreement with banks | PNM | PNM 2020 Term Loan
Debt Instrument [Line Items]
Variable interest rate										2.70%
Term loan agreement with banks | Texas-New Mexico Power Company | PNMR Development Term Loan
Debt Instrument [Line Items]
Aggregate principal amount										$ 90,000,000.0
Variable interest rate										0.98%
San Juan Generating Station | PNM
Debt Instrument [Line Items]
Request issuance of energy transition bonds					$ 361,000,000				$ 361,000,000