Financing - Activities, PNM and TNMP (Details)															1 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Jan. 08, 2020 shares	Jan. 07, 2020 $ / shares shares	Jul. 01, 2019 USD ($)	Apr. 01, 2019 USD ($)	Feb. 26, 2019 USD ($)	Dec. 21, 2018 USD ($)	Dec. 14, 2018 USD ($)	Nov. 26, 2018 USD ($)	Jul. 31, 2018 USD ($)	May 14, 2018 USD ($)	Apr. 09, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 15, 2017	Jul. 28, 2017 USD ($)	Dec. 31, 2018 USD ($) shares	Jul. 31, 2018 USD ($)	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) derivative	Dec. 18, 2019 USD ($)	Mar. 29, 2019 USD ($)	Jan. 18, 2019 USD ($)	Dec. 17, 2018 USD ($)	Jul. 25, 2018 USD ($)	Jun. 28, 2018 USD ($)	May 22, 2018 USD ($)	Mar. 09, 2018 USD ($)	Jul. 20, 2017 USD ($)	Jun. 14, 2017 USD ($)	Jan. 01, 2017 USD ($) loan	Feb. 01, 2016 USD ($)	Mar. 09, 2015 USD ($)
Debt Instrument [Line Items]
Outstanding amount															$ 235,900,000				$ 185,100,000	$ 235,900,000
Common stock, shares issued (in shares) | shares															79,653,624				79,653,624	79,653,624
Interest charges																			$ 121,016,000	$ 127,244,000	$ 127,625,000
Other deferred credits															$ 167,668,000				$ 185,753,000	167,668,000
Fixed interest rate																																1.927%
PNMR 2016 One-Year Term Loan and PNMR 2016 Two-Year Term Loan
Debt Instrument [Line Items]
Number of loan agreements (in loans) | loan																																2
PNMR 2016 One Year Term Loan
Debt Instrument [Line Items]
Outstanding amount																																$ 100,000,000.0
Debt instrument, term													1 year		1 year	1 year
PNMR 2016 Two-Year Term Loan due December 2018
Debt Instrument [Line Items]
Loan entered into																																100,000,000.0
Debt instrument, term															2 years	2 years
PNMR Development Term Loan due November 2020
Debt Instrument [Line Items]
Outstanding amount								$ 90,000,000.0
Variable interest rate																			2.60%
PNMR 2018 One Year Term Loan
Debt Instrument [Line Items]
Aggregate principal amount							$ 150,000,000.0
Outstanding amount															$ 150,000,000				$ 0	150,000,000
Debt instrument, term							1 year
Variable interest rate																			2.70%
PNMR 2018 Two-Year Term Loan due December 2020
Debt Instrument [Line Items]
Aggregate principal amount						$ 50,000,000.0
Debt instrument, term						2 years
Variable interest rate																			2.60%
TNMP 2018 Term Loan Agreement
Debt Instrument [Line Items]
Term loan restatement to provide additional funding																									$ 15,000,000.0
TNMP 2018 Term Loan due July 2020
Debt Instrument [Line Items]
Term loan restatement committed amount																									$ 35,000,000.0
Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Aggregate principal amount																					50,000,000.0
Time hedged in interest rate cash flow hedge																			4 years
Level 2 | Cash Flow Hedging
Debt Instrument [Line Items]
Fair value gain (loss, less than)																			$ 400,000
Fair value gain (loss, less than)																				1,000,000.0
Public Service Company of New Mexico
Debt Instrument [Line Items]
Outstanding amount															$ 42,400,000				$ 58,000,000	$ 42,400,000
Common stock, shares issued (in shares) | shares															39,117,799				39,117,799	39,117,799
Interest charges																			$ 72,900,000	$ 76,458,000	82,697,000
Other deferred credits															$ 215,737,000				155,744,000	215,737,000
Public Service Company of New Mexico | San Juan Generating Station
Debt Instrument [Line Items]
Request issuance of energy transition bonds			$ 361,000,000																$ 361,000,000
Public Service Company of New Mexico | PNM 2019 $250.0 Million Term Loan due July 2020
Debt Instrument [Line Items]
Aggregate principal amount																								$ 250,000,000.0
Public Service Company of New Mexico | PNM 2019 $40.0 Million Term Loan due June 2021
Debt Instrument [Line Items]
Aggregate principal amount																						$ 40,000,000.0
Public Service Company of New Mexico | NMPRC
Debt Instrument [Line Items]
Requirement to obtain approval for any financing transaction, period of time (more than)																			18 months
PNMR | PNMR Development Term Loan due November 2020
Debt Instrument [Line Items]
Loan entered into															90,000,000				$ 90,000,000	90,000,000
PNMR | PNMR 2018 Two-Year Term Loan due December 2020
Debt Instrument [Line Items]
Loan entered into															50,000,000				50,000,000	50,000,000
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply
Debt Instrument [Line Items]
Loan entered into												$ 50,100,000
Repurchased face amount																												$ 50,100,000
NM Capital | San Juan Generating Station | Coal Supply
Debt Instrument [Line Items]
Payments to fund long-term loans to unaffiliated third party												125,000,000.0
Texas-New Mexico Power Company
Debt Instrument [Line Items]
Outstanding amount															$ 17,500,000				$ 15,000,000	$ 17,500,000
Common stock, shares issued (in shares) | shares															6,358				6,358	6,358
Interest charges																			$ 29,100,000	$ 32,091,000	$ 30,084,000
Other deferred credits															$ 2,908,000				$ 7,469,000	2,908,000
Line of Credit
Debt Instrument [Line Items]
Debt-to-capital ratios (less than or equal to)								70.00%									65.00%	65.00%	70.00%
Line of Credit | Public Service Company of New Mexico
Debt Instrument [Line Items]
Debt-to-capital ratios (less than or equal to)																			65.00%
Line of Credit | Texas-New Mexico Power Company
Debt Instrument [Line Items]
Debt-to-capital ratios (less than or equal to)																			65.00%
PNMR 2015 Term Loan Agreement
Debt Instrument [Line Items]
Loan entered into																																150,000,000.0		$ 150,000,000.0
BTMU Term Loan Agreement | NM Capital
Debt Instrument [Line Items]
Loan entered into												$ 125,000,000.0																					$ 125,000,000.0
Repurchased face amount																												$ 43,000,000.0
Estimated principal payments																			$ 43,000,000.0
Letter of Credit | PNMR | Letter or credit, 30 mil JP Morgan
Debt Instrument [Line Items]
Aggregate principal amount																																30,300,000
Senior Unsecured Notes
Debt Instrument [Line Items]
Aggregate principal amount																													$ 300,000,000.0
Stated percentage																													3.25%
Senior Unsecured Notes | Public Service Company of New Mexico | Senior Unsecured Note Agreement (SUNs)
Debt Instrument [Line Items]
Aggregate principal amount														$ 450,000,000.0
Debt to capital ratio														65.00%
Senior Unsecured Notes | Public Service Company of New Mexico | SUNs, Issuance in May 2018
Debt Instrument [Line Items]
Aggregate principal amount										$ 350,000,000.0
Senior Unsecured Notes | Public Service Company of New Mexico | SUN's, Issuance in July 2018
Debt Instrument [Line Items]
Aggregate principal amount									$ 100,000,000.0							$ 100,000,000.0
PNM 2016 Term Loan Agreement due 2017 | Public Service Company of New Mexico
Debt Instrument [Line Items]
Aggregate principal amount																																175,000,000.0
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 2.125 Percent, due 2040
Debt Instrument [Line Items]
Aggregate principal amount																																$ 37,000,000.0
Stated percentage																																2.125%
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 2.45 Percent, due 2042
Debt Instrument [Line Items]
Aggregate principal amount																																$ 20,000,000.0
Stated percentage																																2.45%
Unsecured Debt | Public Service Company of New Mexico
Debt Instrument [Line Items]
Shelf registration statement, capacity of issuance (up to)																			475,000,000.0
Unsecured Debt | Public Service Company of New Mexico | PNM 2017 Term Loan Agreement
Debt Instrument [Line Items]
Loan entered into															200,000,000				0	200,000,000
Unsecured Debt | Public Service Company of New Mexico | SUNs, Issuance in May 2018 | Minimum
Debt Instrument [Line Items]
Debt instrument, term										5 years
Stated percentage										3.15%
Unsecured Debt | Public Service Company of New Mexico | SUNs, Issuance in May 2018 | Maximum
Debt Instrument [Line Items]
Debt instrument, term										30 years
Stated percentage										4.50%
Unsecured Debt | Public Service Company of New Mexico | SUN's, Issuance in July 2018 | Minimum
Debt Instrument [Line Items]
Debt instrument, term									10 years
Stated percentage									3.78%							3.78%
Unsecured Debt | Public Service Company of New Mexico | SUN's, Issuance in July 2018 | Maximum
Debt Instrument [Line Items]
Debt instrument, term									30 years
Stated percentage									4.60%							4.60%
Unsecured Debt | Public Service Company of New Mexico | 7.95% due May 2018
Debt Instrument [Line Items]
Stated percentage										7.95%
Unsecured Debt | Public Service Company of New Mexico | 7.50% due August 2018
Debt Instrument [Line Items]
Stated percentage									7.50%							7.50%
Unsecured Debt | Public Service Company of New Mexico | PNM 2019 $250.0 Million Term Loan due July 2020
Debt Instrument [Line Items]
Loan entered into															0				250,000,000	0
Unsecured Debt | Public Service Company of New Mexico | PNM 2019 $40.0 Million Term Loan due June 2021
Debt Instrument [Line Items]
Loan entered into																			40,000,000
Unsecured Debt | Texas-New Mexico Power Company | 9.50% due April 2019
Debt Instrument [Line Items]
Stated percentage				9.50%
Extinguishment of debt				$ 172,300,000
First Mortgage Bonds | Texas-New Mexico Power Company | Interest Rate of 3.22%, Due 2027
Debt Instrument [Line Items]
Aggregate principal amount																															$ 60,000,000.0
Stated percentage																															3.22%
First Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2018 Term Loan due July 2020
Debt Instrument [Line Items]
Loan entered into															$ 35,000,000				$ 0	35,000,000
First Mortgage Bonds | Texas-New Mexico Power Company | 9.50% due April 2019
Debt Instrument [Line Items]
Stated percentage																			9.50%
First Mortgage Bonds | Texas-New Mexico Power Company | 3.60% due July 2029
Debt Instrument [Line Items]
Stated percentage																			3.60%
Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds
Debt Instrument [Line Items]
Aggregate principal amount																											$ 60,000,000.0
Stated percentage																											3.85%
Mortgages | Texas-New Mexico Power Company | TNMP 2019 Bond Purchase Agreement
Debt Instrument [Line Items]
Aggregate principal amount					$ 305,000,000.0																		$ 225,000,000.0
Stated percentage					400.00%
Debt to capital ratio					65.00%
Mortgages | Texas-New Mexico Power Company | 3.60% due July 2029
Debt Instrument [Line Items]
Aggregate principal amount			$ 80,000,000.0
Secured Debt | Texas-New Mexico Power Company | TNMP Term Loan Agreement
Debt Instrument [Line Items]
Aggregate principal amount																										$ 20,000,000.0
JPMorgan Chase Bank, N.A. and U.S. Bank National Association | PNM 2016 Term Loan Agreement due 2017 | Public Service Company of New Mexico
Debt Instrument [Line Items]
Aggregate principal amount																														$ 175,000,000.0
JPMorgan Chase Bank, N.A. and U.S. Bank National Association | Notes Payable to Banks | Public Service Company of New Mexico | PNM 2017 Term Loan Agreement
Debt Instrument [Line Items]
Aggregate principal amount				$ 200,000,000.0
JPMorgan Chase Bank, N.A. and U.S. Bank National Association | Notes Payable to Banks | Public Service Company of New Mexico | PNM 2019 $250.0 Million Term Loan due July 2020
Debt Instrument [Line Items]
Stated percentage																			2.45%
JPMorgan Chase Bank, N.A. and U.S. Bank National Association | Notes Payable to Banks | Public Service Company of New Mexico | PNM 2019 $40.0 Million Term Loan due June 2021
Debt Instrument [Line Items]
Stated percentage																			2.39%
Interest rate contract
Debt Instrument [Line Items]
Number of derivative instruments held | derivative																					3
Interest rate contract one | Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Fixed interest rate																					1.926%
Interest rate contract two | Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Fixed interest rate																					1.823%
Interest rate contract three | Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Fixed interest rate																					1.629%
Deposit Related to Potential Transmission Interconnections | PNMR Development
Debt Instrument [Line Items]
Amount of related party transaction											$ 68,200,000
Accrued interest											5,700,000
Interest charges																			$ 3,300,000	$ 2,400,000
Other deferred credits											$ 68,200,000
PNMR 2020 Forward Equity Sales Agreement | Subsequent Event
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 | Subsequent Event
Debt Instrument [Line Items]
Number of shares issued in transaction (in shares) | shares	800,000	800,000