Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)	1 Months Ended			4 Months Ended	9 Months Ended	13 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2012 landowner	Apr. 30, 2010 city	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) generating_unit	Jun. 30, 2020 USD ($)	Dec. 01, 2015 Allotment_Parcel	Jul. 13, 2015 a	Jan. 22, 2015 Allotment_Parcel	Aug. 31, 2013
Continuous highwall mining | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Proposed retroactive surface mining royalty rate										12.50%
Surface mining royalty rate applied	8.00%			8.00%	8.00%
Estimated underpaid surface mining royalties under proposed rate change	$ 5,000,000			$ 5,000,000	$ 5,000,000
PNM's share estimated underpaid surface mining royalties under proposed rate change	46.30%			46.30%	46.30%
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Franchise fee agreement term					15 years
PNM | Navajo Nation Allottee Matters
Public Utilities, Commitments And Contingencies [Line Items]
Number of landowners involved in the appeal | landowner		43
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel							2		10
Area of land (in acres) | a								15.49
Number of allotment parcels at issue that are not to be condemned | Allotment_Parcel							2
Number of allotment parcels at issue | Allotment_Parcel							5
PNM | Palo Verde Nuclear Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Number of cities to provide cooling water | city			5
Term of agreement for cooling water			40 years
PNM | Palo Verde Nuclear Generating Station | Nuclear plant
Public Utilities, Commitments And Contingencies [Line Items]
Ownership percentage in nuclear reactor	10.20%			10.20%	10.20%
Number of units | generating_unit					3
Maximum potential assessment per incident	$ 42,100,000			$ 42,100,000	$ 42,100,000
Annual payment limitation related to incident	6,200,000			6,200,000	6,200,000
Aggregate amount of all risk insurance	2,800,000,000			2,800,000,000	2,800,000,000
Maximum amount under Nuclear Electric Insurance Limited	5,400,000			5,400,000	5,400,000
Asset retirement obligation, revision of estimate	2,800,000			9,200,000		$ 6,400,000
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage	13,800,000,000			13,800,000,000	13,800,000,000
Liability insurance coverage sublimit	2,250,000,000			2,250,000,000	2,250,000,000
Commercial providers | PNM | Palo Verde Nuclear Generating Station | Nuclear plant
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage	450,000,000			450,000,000	450,000,000
Industry Wide Retrospective Assessment Program | PNM | Palo Verde Nuclear Generating Station | Nuclear plant
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage	$ 13,300,000,000			$ 13,300,000,000	$ 13,300,000,000
Pending litigation
Public Utilities, Commitments And Contingencies [Line Items]
Written notification to terminate agreement, minimum period of time required					30 days