Financing - Long-term Debt (Details) - USD ($)	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 21, 2016	Dec. 17, 2015	Aug. 11, 2015
Debt Instrument [Line Items]
Total	$ 2,431,507,000		$ 2,388,577,000
Principal, less current maturities	257,293,000		274,025,000
Long-term debt, excluding current maturities, gross	2,174,214,000		2,114,552,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	(6,138,000)		(4,135,000)
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities	398,000		677,000
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities	(6,536,000)		(4,812,000)
PNMR 2016 Two-Year Term Loan due December 2018
Debt Instrument [Line Items]
Long-term debt				$ 100,000,000.0
Public Service Company of New Mexico
Debt Instrument [Line Items]
Total	1,665,870,000		1,640,870,000
Principal, less current maturities	25,000		232,000,000
Long-term debt, excluding current maturities, gross	1,665,845,000		1,408,870,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	7,960,000		9,501,000
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities	2,000		120,000
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities	$ 7,958,000		$ 9,381,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 1.875% due April 2033, mandatory tender - October 1, 2021
Debt Instrument [Line Items]
Stated percentage	1.875%		1.875%
Unsecured long-term debt, noncurrent	$ 146,000,000		$ 146,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 1,383,000		$ 1,807,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 6.25% due January 2038
Debt Instrument [Line Items]
Stated percentage	6.25%		6.25%
Unsecured long-term debt, noncurrent	$ 36,000,000		$ 36,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	228,000		$ 239,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 4.75% due June 2040, mandatory tender - June 1, 2017
Debt Instrument [Line Items]
Stated percentage			4.75%
Unsecured long-term debt, noncurrent	0		$ 37,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 0		25,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.125% due June 2040, mandatory tender - June 1, 2022
Debt Instrument [Line Items]
Stated percentage	2.125%
Unsecured long-term debt, noncurrent	$ 37,000,000		0
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 404,000		$ 0
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.20% due June 2040, mandatory tender - June 1, 2020
Debt Instrument [Line Items]
Stated percentage	5.20%		5.20%
Unsecured long-term debt, noncurrent	$ 40,045,000		$ 40,045,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 105,000		$ 147,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.90% due June 2040
Debt Instrument [Line Items]
Stated percentage	5.90%		5.90%
Unsecured long-term debt, noncurrent	$ 255,000,000		$ 255,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 2,040,000		$ 2,131,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 6.25% due June 2040
Debt Instrument [Line Items]
Stated percentage	6.25%		6.25%
Unsecured long-term debt, noncurrent	$ 11,500,000		$ 11,500,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	92,000		$ 96,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.54% due September 2042, mandatory tender - June 1, 2017
Debt Instrument [Line Items]
Stated percentage			2.54%
Unsecured long-term debt, noncurrent	0		$ 20,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 0		67,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.45% due September 2042, mandatory tender - June 1, 2022
Debt Instrument [Line Items]
Stated percentage	2.45%
Unsecured long-term debt, noncurrent	$ 20,000,000		0
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 153,000		$ 0
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.40% due June 2043, mandatory tender - June 1, 2020
Debt Instrument [Line Items]
Stated percentage	2.40%		2.40%
Unsecured long-term debt, noncurrent	$ 39,300,000		$ 39,300,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 243,000		$ 340,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.20% due June 2043, mandatory tender - June 1, 2020
Debt Instrument [Line Items]
Stated percentage	5.20%		5.20%
Unsecured long-term debt, noncurrent	$ 21,000,000		$ 21,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 53,000		$ 75,000
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 7.95% due May 2018
Debt Instrument [Line Items]
Stated percentage	7.95%		7.95%
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 7.50% due August 2018
Debt Instrument [Line Items]
Stated percentage	7.50%		7.50%
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.35% due October 2021
Debt Instrument [Line Items]
Stated percentage	5.35%		5.35%
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 3.85% due August 2025
Debt Instrument [Line Items]
Stated percentage	3.85%		3.85%
Public Service Company of New Mexico | Senior Unsecured Notes: | 7.95% due May 2018
Debt Instrument [Line Items]
Stated percentage	7.95%
Unsecured long-term debt, noncurrent	$ 350,000,000	$ 450,000,000.0	$ 350,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 272,000		995,000
Public Service Company of New Mexico | Senior Unsecured Notes: | 7.50% due August 2018
Debt Instrument [Line Items]
Stated percentage	7.50%
Unsecured long-term debt, noncurrent	$ 100,025,000		100,025,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	73,000		197,000
Public Service Company of New Mexico | Senior Unsecured Notes: | 5.35% due October 2021
Debt Instrument [Line Items]
Unsecured long-term debt, noncurrent	160,000,000		160,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	617,000		780,000
Public Service Company of New Mexico | Senior Unsecured Notes: | 3.85% due August 2025
Debt Instrument [Line Items]
Stated percentage						3.85%
Unsecured long-term debt, noncurrent	250,000,000		250,000,000			$ 250,000,000.0
Unamortized Discounts, (Premiums), and Issuance Costs, net	2,274,000		2,574,000
Public Service Company of New Mexico | Senior Unsecured Notes: | PNM 2016 Term Loan Agreement due November 2017
Debt Instrument [Line Items]
Long-term debt	0		175,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	0		28,000
Public Service Company of New Mexico | Senior Unsecured Notes: | PNM 2017 Term Loan Agreement due January 2019
Debt Instrument [Line Items]
Long-term debt	200,000,000		0
Unamortized Discounts, (Premiums), and Issuance Costs, net	23,000		0
Texas-New Mexico Power Company
Debt Instrument [Line Items]
Total	465,500,000		405,500,000
Principal, less current maturities	0		0
Long-term debt, excluding current maturities, gross	465,500,000		405,500,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	(15,120,000)		(15,375,000)
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities	0		0
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities	$ (15,120,000)		$ (15,375,000)
Texas-New Mexico Power Company | First Mortgage Bonds: | 9.50% due April 2019
Debt Instrument [Line Items]
Stated percentage	9.50%		9.50%
Unsecured long-term debt, noncurrent	$ 172,302,000		$ 172,302,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 1,032,000		$ 1,857,000
Texas-New Mexico Power Company | First Mortgage Bonds: | 6.95% due April 2043
Debt Instrument [Line Items]
Stated percentage	6.95%		6.95%
Unsecured long-term debt, noncurrent	$ 93,198,000		$ 93,198,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ (18,057,000)		$ (18,773,000)
Texas-New Mexico Power Company | First Mortgage Bonds: | 4.03% due July 2024
Debt Instrument [Line Items]
Stated percentage	4.03%		4.03%
Unsecured long-term debt, noncurrent	$ 80,000,000		$ 80,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 686,000		$ 792,000
Texas-New Mexico Power Company | First Mortgage Bonds: | 3.53% due February 2026
Debt Instrument [Line Items]
Stated percentage	3.53%		3.53%		3.53%
Unsecured long-term debt, noncurrent	$ 60,000,000		$ 60,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 667,000		749,000
Texas-New Mexico Power Company | First Mortgage Bonds: | 3.22% due August 2027
Debt Instrument [Line Items]
Stated percentage	3.22%
Unsecured long-term debt, noncurrent	$ 60,000,000		0
Unamortized Discounts, (Premiums), and Issuance Costs, net	552,000		0
PNMR
Debt Instrument [Line Items]
Total	300,137,000		342,207,000
Principal, less current maturities	257,268,000		42,025,000
Long-term debt, excluding current maturities, gross	42,869,000		300,182,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	1,022,000		1,739,000
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities	396,000		557,000
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities	626,000		1,182,000
PNMR | PNMR 2015 Term Loan Agreement due March 2018
Debt Instrument [Line Items]
Long-term debt	150,000,000		150,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	12,000		84,000
PNMR | BTMU Term Loan Agreement, payments through February 2021
Debt Instrument [Line Items]
Long-term debt	50,137,000		92,207,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	1,001,000		1,634,000
PNMR | PNMR 2016 Two-Year Term Loan due December 2018
Debt Instrument [Line Items]
Long-term debt	100,000,000		100,000,000
Unamortized Discounts, (Premiums), and Issuance Costs, net	$ 9,000		$ 21,000