Commitments and Contingencies - The Clean Air Act (Details) $ in Millions		9 Months Ended
	Mar. 02, 2015 lb / MMBTU T	Sep. 30, 2018 USD ($) lbsofnox / mmbtu option joint_owner T	May 29, 2018 parts_per_billion	Dec. 31, 2017 MW	Jan. 26, 2016 state	Dec. 31, 2015 MW	Oct. 01, 2015 parts_per_billion	Sep. 30, 2015 parts_per_billion	May 14, 2015 lb / MMBTU	Dec. 30, 2013	Apr. 30, 2013 well	Feb. 28, 2013 lawsuit state mine	Aug. 06, 2012 compliance_alternative	Dec. 31, 1999 state
Public Utilities, Commitments And Contingencies [Line Items]
Undepreciated investment in ownership to be obtained		$ 405.5
Number of options for meeting BTA standards | option		7
Expected number of additional monitoring wells		38
Clean Air Act related to regional haze
Public Utilities, Commitments And Contingencies [Line Items]
Number of states to address regional haze (in states) | state														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 | state					29
San Juan Generating Station Unit 4 | Clean Air Act, SNCR
Public Utilities, Commitments And Contingencies [Line Items]
Number of other joint owners | joint_owner		8
PNM | National Ambient Air Quality Standards, 2015 EPA Legal Settlement
Public Utilities, Commitments And Contingencies [Line Items]
Period of time to act on settlement	16 months
Emissions tons of SO2 per year (more than) | T	16,000
PNM | WEG v OSM lawsuit
Public Utilities, Commitments And Contingencies [Line Items]
Number of mines impacted | mine												7
Number of states impacted | state												4
Number of claims for relief, filed | lawsuit												15
PNM | Santa Fe generating station
Public Utilities, Commitments And Contingencies [Line Items]
Number of wells with elevated levels of nitrates | well											3
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR Hearing Examiner, Recommended Denial
Public Utilities, Commitments And Contingencies [Line Items]
Additional ownership to be obtained (in megawatts) | MW						132
Undepreciated interest in ownership to be obtained		$ 20.3
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR
Public Utilities, Commitments And Contingencies [Line Items]
Additional ownership to be obtained (in megawatts) | MW						65
Undepreciated interest in ownership to be obtained		$ 10.1
PNM | Palo Verde Nuclear Generating Station Unit 3 | Clean Air Act, SNCR
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts nuclear generation (in megawatts) | MW				134
PNM | Four Corners | Clean Air Act related to post combustion controls
Public Utilities, Commitments And Contingencies [Line Items]
Number of compliance alternatives | compliance_alternative													2
Plant requirement to meet NOx emissions limit (in pounds of NOx per MMBTU) | lbsofnox / mmbtu		0.015
Plant requirement to meet opacity limit		20.00%
Rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations		20.00%
Estimate of possible loss		$ 88.6
PNM | Four Corners Units 4 and 5 (Coal) | Clean Air Act related to post combustion controls
Public Utilities, Commitments And Contingencies [Line Items]
Ownership percentage		13.00%
PNM | Four Corners Units 1, 2 and 3 (Coal) | Clean Air Act related to post combustion controls
Public Utilities, Commitments And Contingencies [Line Items]
Ownership percentage										0.00%
PNM | San Juan Generating Station | WEG v OSM lawsuit
Public Utilities, Commitments And Contingencies [Line Items]
Number of claims for relief, filed | lawsuit												2
Minimum | PNM | National Ambient Air Quality Standards, 2015 EPA Legal Settlement
Public Utilities, Commitments And Contingencies [Line Items]
Emissions tons of SO2 per year (more than) | T	2,600
One-hour SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU	0.45
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				70	75
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 / MMBTU									0.10