Commitments and Contingencies - Coal Supply (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2019	Jun. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2020	Jan. 31, 2016
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets	$ 44,472				$ 50,070
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets	36,561				37,661
PNM | Loss on long-term purchase commitment | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Final reclamation, capped amount to be collected				$ 100,000
Loss contingency accrual	70,300				69,500
PNM | Loss on long-term purchase commitment | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual	25,300				26,900
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss					89,500
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss					40,000
Coal supply | PNM | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets	26,300				26,300
Coal supply | NM Capital | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Requirement to post reclamation bonds					118,700
Cash used to support bank letter or credit arrangement					30,300	$ 30,300
Increase in coal mine decommissioning liability | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded		$ 300	$ 1,100
Increase in coal mine decommissioning liability | PNM | Underground and Surface
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded				39,200
Increase in coal mine decommissioning liability | PNM | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded				$ 9,400
Mine Reclamation Trust | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required contribution to reclamation trust, current fiscal year					8,100
Reclamation trust funding, year 2					10,900
Reclamation trust funding, year 3					11,700
Four Corners CSA | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Other commitment, due in first year					2,100
Other commitment, due in second year					1,900
Other commitment, due in third year					1,900
Other commitment, due in fourth year					1,900
Other commitment, due in fifth year					$ 1,900
San Juan Generating Station | Loss on long-term purchase commitment | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Annual funding post-term reclamation trust	$ 5,500