Commitments and Contingencies - Coal Supply (Details)				3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 01, 2016 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Oct. 20, 2017 USD ($)	Jun. 30, 2017	Feb. 01, 2017	Aug. 01, 2016 payment
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets				$ 56,421,000		$ 56,421,000		$ 73,444,000
Regulatory disallowances and restructuring costs				0	$ 16,451,000	0	$ 17,225,000
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets				48,883,000		48,883,000		67,355,000
Regulatory disallowances and restructuring costs				0	16,451,000	0	$ 17,225,000
PNM | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory assets				100,000,000		100,000,000
PNM | Loss on long-term purchase commitment | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual				41,400,000		41,400,000		41,000,000
Final reclamation, capped amount to be collected						100,000,000.0
PNM | Loss on long-term purchase commitment | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual				14,700,000		14,700,000		14,000,000
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs			$ 16,500,000
Estimate of possible loss				100,900,000		100,900,000
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss				127,400,000		127,400,000
Coal supply | PNM | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets				31,900,000		31,900,000		48,700,000
Estimated increase in coal cost										51.00%
Coal supply | NM Capital | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Payments to fund long-term loans to unaffiliated third party		$ 125,000,000.0
Loan agreement among several entities		$ 125,000,000.0
Interest rate		7.25%									9.25%
Requirement to post reclamation bonds				118,700,000		118,700,000
Cash used to support bank letter or credit arrangement		$ 30,300,000		30,300,000		30,300,000
Coal supply | NM Capital | San Juan Coal Company, Westmoreland
Public Utilities, Commitments And Contingencies [Line Items]
Payments to fund long-term loans to unaffiliated third party	$ 125,000,000.0
Loan receivable		$ 125,000,000		66,200,000		66,200,000
Scheduled principal payment proceeds				9,600,000		9,600,000
Scheduled interest payment proceeds				1,800,000		1,800,000
Prepayment penalty				0		0
BTMU Term Loan Agreement | NM Capital
Public Utilities, Commitments And Contingencies [Line Items]
Long-term Debt	$ 125,000,000.0			60,900,000		60,900,000
Subsequent event | Coal supply | Unaffiliated third party | San Juan Coal Company, Westmoreland
Public Utilities, Commitments And Contingencies [Line Items]
Amount held in a restricted bank account									$ 11,400,000
Increase in coal mine decommissioning liability | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Expense reflected in regulatory disallowances and restructuring costs					$ 4,800,000			$ 4,500,000
Mine Reclamation Trust | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required contribution to Reclamation Trust, current fiscal year				5,800,000		5,800,000
Reclamation Trust Funding, Year 2				8,300,000		8,300,000
Reclamation Trust Funding, Year 3				8,700,000		8,700,000
Four Corners | Mine Reclamation Trust | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required contribution to Reclamation Trust, current fiscal year				2,300,000		2,300,000
Reclamation Trust Funding, Year 2				2,100,000		2,100,000
Reclamation Trust Funding, Year 3				$ 2,100,000		$ 2,100,000
Number of annual installment payments | payment												13