Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)	1 Months Ended		3 Months Ended
	Sep. 30, 2012 landowner	Apr. 30, 2010 city	Mar. 31, 2020 USD ($) generating_unit	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%
Estimated underpaid surface mining royalties under proposed rate change			$ 5,000,000
PNM's share estimated underpaid surface mining royalties under proposed rate change			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%
Number of units | generating_unit			3
Maximum potential assessment per incident			$ 42,100,000
Annual payment limitation related to incident			6,200,000
Aggregate amount of all risk insurance			2,800,000,000
Maximum amount under Nuclear Electric Insurance Limited			5,400,000
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage			13,900,000,000
Liability insurance coverage sublimit			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
Industry Wide Retrospective Assessment Program | PNM | Palo Verde Nuclear Generating Station | Nuclear plant
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage			$ 13,500,000,000
Pending litigation
Public Utilities, Commitments And Contingencies [Line Items]
Written notification to terminate agreement, minimum period of time required			30 days