Financing - Financing Activities (Details) - USD ($)				1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 09, 2018	Jul. 28, 2017	Feb. 01, 2016	Jul. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jul. 25, 2018	Jun. 28, 2018	May 22, 2018	May 14, 2018	Mar. 09, 2018	Dec. 31, 2017	Jul. 20, 2017	Oct. 21, 2016	Mar. 09, 2015
Debt Instrument [Line Items]
Interest Charges					$ 30,492,000	$ 32,106,000	$ 96,868,000	$ 96,137,000
Other deferred credits					127,612,000		$ 127,612,000							$ 131,706,000
PNMR 2016 One Year Term Loan
Debt Instrument [Line Items]
Term of loan				1 year			1 year
PNMR 2016 Two-Year Term Loan
Debt Instrument [Line Items]
Term of loan				2 years			2 years
Term loans					$ 100,000,000.0		$ 100,000,000.0
Variable interest rate					3.03%		3.03%
PNM
Debt Instrument [Line Items]
Interest Charges					$ 18,063,000	20,451,000	$ 58,881,000	62,393,000
Other deferred credits					171,345,000		$ 171,345,000							106,442,000
PNM | Maximum
Debt Instrument [Line Items]
Maturity term over which financings require regulator approval (more than)							18 months
NM Capital | Coal supply | San Juan Coal Company, Westmoreland
Debt Instrument [Line Items]
Repayment of line of credit											$ 50,100,000
Payments to fund long-term loans to unaffiliated third party			$ 125,000,000.0
Texas-New Mexico Power Company
Debt Instrument [Line Items]
Interest Charges					8,241,000	$ 7,704,000	$ 23,771,000	$ 22,619,000
Other deferred credits					6,692,000		6,692,000							7,448,000
Repayments from transmission interconnection arrangement					4,100,000		$ 4,100,000							$ 0
Line of credit
Debt Instrument [Line Items]
Ratio of debt to capital (less than or equal to)				70.00%			65.00%
BTMU Term Loan Agreement | NM Capital
Debt Instrument [Line Items]
Repayment of line of credit			125,000,000.0								$ 43,000,000
Term loans			$ 125,000,000.0
Letter of credit | PNMR | JPM LOC Facility
Debt Instrument [Line Items]
Aggregate principal amount																$ 30,300,000.0
Senior Unsecured Notes
Debt Instrument [Line Items]
Aggregate principal amount													$ 300,000,000.0
Stated interest rate													3.25%
Senior Unsecured Notes | PNM | Senior Unsecured Note Agreement (SUNs)
Debt Instrument [Line Items]
Aggregate principal amount		$ 450,000,000.0			450,000,000		$ 450,000,000
Debt-to-Capital ratio		65.00%
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018
Debt Instrument [Line Items]
Aggregate principal amount												$ 350,000,000.0
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018 | Plan
Debt Instrument [Line Items]
Aggregate principal amount					350,000,000		350,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018
Debt Instrument [Line Items]
Aggregate principal amount				$ 100,000,000.0
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018 | Plan
Debt Instrument [Line Items]
Aggregate principal amount					100,000,000		100,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.15%
Debt Instrument [Line Items]
Aggregate principal amount					$ 55,000,000.0		$ 55,000,000.0
Stated interest rate					3.15%		3.15%
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.45%
Debt Instrument [Line Items]
Aggregate principal amount					$ 104,000,000		$ 104,000,000
Stated interest rate					3.45%		3.45%
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.68%
Debt Instrument [Line Items]
Aggregate principal amount					$ 88,000,000		$ 88,000,000
Stated interest rate					3.68%		3.68%
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.93%
Debt Instrument [Line Items]
Aggregate principal amount					$ 38,000,000		$ 38,000,000
Stated interest rate					3.93%		3.93%
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.22%
Debt Instrument [Line Items]
Aggregate principal amount					$ 45,000,000		$ 45,000,000
Stated interest rate					4.22%		4.22%
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.50%
Debt Instrument [Line Items]
Aggregate principal amount					$ 20,000,000		$ 20,000,000
Stated interest rate					4.50%		4.50%
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 3.78%
Debt Instrument [Line Items]
Aggregate principal amount					$ 15,000,000		$ 15,000,000
Stated interest rate					3.78%		3.78%
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 4.60%
Debt Instrument [Line Items]
Aggregate principal amount					$ 85,000,000		$ 85,000,000
Stated interest rate					4.60%		4.60%
PNMR 2015 Term Loan Agreement
Debt Instrument [Line Items]
Term loans																	$ 150,000,000.0
Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds
Debt Instrument [Line Items]
Aggregate principal amount					$ 172,300,000.0		$ 172,300,000.0			$ 60,000,000.0
Stated interest rate									2.94%	3.85%
TNMP Term Loan Agreement | Texas-New Mexico Power Company | TNMP Term Loan Agreement
Debt Instrument [Line Items]
Aggregate principal amount									$ 20,000,000.0
PNM 2017 Term Loan Agreement
Debt Instrument [Line Items]
Interest rate at period end					2.98%		2.98%
Term loan agreement with banks | PNM | PNM 2017 Term Loan Agreement | JPMorgan Chase Bank, N.A. and U.S. Bank National Association
Debt Instrument [Line Items]
Aggregate principal amount															$ 200,000,000.0
Term loan agreement with banks | Texas-New Mexico Power Company | TNMP Term Loan Agreement | JPMorgan Chase Bank, N.A. and U.S. Bank National Association
Debt Instrument [Line Items]
Aggregate principal amount															$ 20,000,000.0
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in May 2018, 7.95%
Debt Instrument [Line Items]
Stated interest rate												7.95%
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in August 2018, 7.50%
Debt Instrument [Line Items]
Stated interest rate					7.50%		7.50%
Deposit Related To Potential Transmission Interconnections | PNMR Development
Debt Instrument [Line Items]
Amount of related party transaction	$ 68,200,000
Interest Charges					$ 800,000		$ 1,500,000
Other deferred credits	$ 68,200,000