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Commitments and Contingencies - The Clean Air Act (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) option T	Jul. 01, 2019 USD ($)	Feb. 25, 2019 parts_per_billion	Dec. 31, 2018 USD ($) MW	Feb. 09, 2016 state	Oct. 01, 2015 parts_per_billion	Sep. 30, 2015 parts_per_billion	May 14, 2015 lb / MMBTU	Feb. 28, 2013 claim state mine	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) | 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
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 | claim									15
PNM | Santa Fe generating station
Public Utilities, Commitments And Contingencies [Line Items]
Estimated liability, abatement activities				$ 0.2
PNM | San Juan Generating Station Unit 4 | Clean Air Act, SNCR
Public Utilities, Commitments And Contingencies [Line Items]
Net book value	$ 345.5			373.6
Pre-tax impairment of investments				35.0
Undepreciated investment in ownership to be obtained				11.9
Forecasted undepreciated investment				$ 23.1
PNM | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Forecasted undepreciated investment		$ 283.0
PNM | San Juan Generating Station | Clean Air Act, SNCR
Public Utilities, Commitments And Contingencies [Line Items]
Additional ownership to be obtained (in megawatts) | MW				132
PNM | San Juan Generating Station | WEG v OSM lawsuit
Public Utilities, Commitments And Contingencies [Line Items]
Number of claims for relief, filed | claim									2
PNMR Development | San Juan Generating Station Unit 4 | Clean Air Act, SNCR
Public Utilities, Commitments And Contingencies [Line Items]
Potential acquisition of ownership (in megawatts) | MW				65
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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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