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Financing - Financing Activities (Details) - USD ($)			3 Months Ended
	Jul. 28, 2017	Feb. 01, 2016	Mar. 31, 2018	Mar. 09, 2018	Jul. 20, 2017	Oct. 21, 2016	Mar. 09, 2015
Debt Instrument [Line Items]
Estimated principal payments			$ 10,000,000
PNMR 2016 Two-Year Term Loan
Debt Instrument [Line Items]
Term loans			$ 100,000,000.0
Term of loan			2 years
Variable interest rate			2.64%
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]
Payments to fund long-term loans to unaffiliated third party		$ 125,000,000.0
Line of credit
Debt Instrument [Line Items]
Ratio of debt to capital (less than or equal to)			65.00%
PNMR 2015 Term Loan Agreement
Debt Instrument [Line Items]
Term loans							$ 150,000,000.0
BTMU Term Loan Agreement | NM Capital
Debt Instrument [Line Items]
Term loans		$ 125,000,000.0	$ 45,100,000
Estimated principal payments			$ 10,000,000
Letter of credit | PNMR | JPM LOC Facility
Debt Instrument [Line Items]
Aggregate principal amount						$ 30,300,000.0
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
Senior Unsecured Notes
Debt Instrument [Line Items]
Stated interest rate				3.25%
Aggregate principal amount				$ 300,000,000
Senior Unsecured Notes | PNM | 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 | PNM | SUNs, Issuance in May 2018 | Plan
Debt Instrument [Line Items]
Aggregate principal amount	$ 350,000,000.0
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018 | Plan
Debt Instrument [Line Items]
Aggregate principal amount	$ 100,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.15%
Debt Instrument [Line Items]
Stated interest rate	3.15%
Aggregate principal amount	$ 55,000,000.0
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.45%
Debt Instrument [Line Items]
Stated interest rate	3.45%
Aggregate principal amount	$ 104,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.68%
Debt Instrument [Line Items]
Stated interest rate	3.68%
Aggregate principal amount	$ 88,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.93%
Debt Instrument [Line Items]
Stated interest rate	3.93%
Aggregate principal amount	$ 38,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.22%
Debt Instrument [Line Items]
Stated interest rate	4.22%
Aggregate principal amount	$ 45,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.50%
Debt Instrument [Line Items]
Stated interest rate	4.50%
Aggregate principal amount	$ 20,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 3.78%
Debt Instrument [Line Items]
Stated interest rate	3.78%
Aggregate principal amount	$ 15,000,000
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 4.60%
Debt Instrument [Line Items]
Stated interest rate	4.60%
Aggregate principal amount	$ 85,000,000
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in May 2018, 7.95%
Debt Instrument [Line Items]
Stated interest rate			7.95%
Amount of senior unsecured notes to be repaid with SUN proceeds			$ 350,000,000
Senior Unsecured Notes			$ 450,000,000
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in August 2018, 7.50%
Debt Instrument [Line Items]
Stated interest rate			7.50%
Senior Unsecured Notes			$ 100,000,000.0
PNM 2017 Term Loan Agreement
Debt Instrument [Line Items]
Interest rate at period end			2.61%
LIBOR | BTMU Term Loan Agreement | NM Capital
Debt Instrument [Line Items]
Stated interest rate			4.41%

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