Financing - Borrowing Arrangements Between PNMR and Its Subsidiaries and Short-term Debt (Details)	Nov. 01, 2020 USD ($)	Feb. 26, 2018 USD ($)	Feb. 20, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 12, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 21, 2016 USD ($)	Nov. 30, 2016 USD ($)	Jan. 08, 2014 USD ($) bank
Short-term Debt [Line Items]
Short-term debt				$ 305,400,000		$ 287,100,000
Letters of credit outstanding				6,400,000
JPMorgan Chase Bank, N.A.
Short-term Debt [Line Items]
Letters of credit outstanding				30,300,000
PNMR 2016 One Year Term Loan
Short-term Debt [Line Items]
Short-term debt				100,000,000		100,000,000	$ 100,000,000.0
PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity				300,000,000				$ 10,000,000.0
Short-term debt				165,600,000		126,100,000
PNMR Revolving Credit Facility | Scenario, forecast
Short-term Debt [Line Items]
Financing capacity	$ 290,000,000.0
Subsequent Event
Short-term Debt [Line Items]
Invested cash			$ 900,000
Subsequent Event | PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity			111,200,000
Subsequent Event | Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity			522,300,000
PNM Resources
Short-term Debt [Line Items]
Short-term debt				$ 265,600,000		226,100,000
PNM Resources | PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Interest rates on outstanding borrowings				2.76%
Texas-New Mexico Power Company
Short-term Debt [Line Items]
Subsidiary loan agreements				$ 0		4,600,000
Letters of credit outstanding				100,000
Texas-New Mexico Power Company | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity				75,000,000
Short-term debt				0		0
Texas-New Mexico Power Company | TNMP Revolving Credit Facility | 9.50% due April 2019
Short-term Debt [Line Items]
Aggregate principal amount of bonds				75,000,000
Texas-New Mexico Power Company | Subsequent Event
Short-term Debt [Line Items]
Subsidiary loan agreements			0
Invested cash			0
Texas-New Mexico Power Company | Subsequent Event | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity			51,000,000
Public Service Company of New Mexico
Short-term Debt [Line Items]
Subsidiary loan agreements				0		0
Short-term debt				39,800,000		61,000,000
Letters of credit outstanding				2,500,000
Public Service Company of New Mexico | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Financing capacity					$ 40,000,000.0
Public Service Company of New Mexico | PNM 2014 New Mexico Credit Facility
Short-term Debt [Line Items]
Financing capacity									$ 50,000,000.0
Number of participating lenders (in banks) | bank									8
Public Service Company of New Mexico | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity				$ 400,000,000				$ 40,000,000.0
Interest rates on outstanding borrowings				2.59%
Short-term debt				$ 39,800,000		35,000,000
Public Service Company of New Mexico | PNM Revolving Credit Facility | Scenario, forecast
Short-term Debt [Line Items]
Financing capacity	$ 360,000,000.0
Public Service Company of New Mexico | PNM 2014 New Mexico Credit Facility
Short-term Debt [Line Items]
Short-term debt				0		26,000,000
Public Service Company of New Mexico | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Short-term debt				0		$ 0
Public Service Company of New Mexico | Subsequent Event
Short-term Debt [Line Items]
Subsidiary loan agreements			0
Invested cash			0
Public Service Company of New Mexico | Subsequent Event | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity			340,100,000
Public Service Company of New Mexico | Subsequent Event | PNM 2014 New Mexico Credit Facility
Short-term Debt [Line Items]
Available borrowing capacity			$ 20,000,000
PNMR Development | Subsequent Event | PNMR 2018 Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity		$ 24,500,000.0
Maximum
Short-term Debt [Line Items]
Subsidiary loan agreements				$ 100,000,000