Commitments and Contingencies - The Clean Air Act (Details)	9 Months Ended
	Sep. 30, 2020 option T	Feb. 25, 2019 parts_per_billion	Mar. 31, 2016 parts_per_billion	Feb. 09, 2016 state	Oct. 01, 2015 parts_per_billion	May 14, 2015 lb_PER_MMBTU	Dec. 31, 1999 state
Public Utilities, Commitments And Contingencies [Line Items]
Number of options for meeting BTA standards | option	7
Clean Air Act related to regional haze
Public Utilities, Commitments And Contingencies [Line Items]
Number of states to address regional haze (in states)							50
Potential to emit tons per year of visibility impairing pollution (in tons, more than) | T	250
Clean Power Plan
Public Utilities, Commitments And Contingencies [Line Items]
Number of states that filed a petition against the Clean Power Plan				29
Maximum | PNM | San Juan Generating Station and Four Corners
Public Utilities, Commitments And Contingencies [Line Items]
Government standard emission limit (in ozone parts per million) | parts_per_billion		75	75		70
San Juan Generating Station | PNM | National Ambient Air Quality Standards
Public Utilities, Commitments And Contingencies [Line Items]
Revised SO2 emissions (in pounds per MMBTU) | lb_PER_MMBTU						0.10