Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 01, 2015 Allotment_Parcel	Jul. 13, 2015 a	Jan. 22, 2015 Allotment_Parcel	Apr. 02, 2014 landowner Allotment_Parcel	Jan. 06, 2014 Allotment_Parcel	Aug. 31, 2013	May 23, 2013 USD ($)	Sep. 30, 2012 landowner
Palo Verde Nuclear Generating Station [Member] | Public Service Company of New Mexico [Member] | Nuclear Plant [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Ownership percentage in nuclear reactor	10.20%
Maximum potential assessment per incident	$ 38,900,000.0
Annual payment limitation related to incident	5,800,000.0
Aggregate amount of all risk insurance	2,750,000,000.00
Maximum amount under Nuclear Electric Insurance Limited	5,400,000.0
Palo Verde Nuclear Generating Station [Member] | Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Commercial Providers [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage	375,000,000
Palo Verde Nuclear Generating Station [Member] | Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Industry Wide Retrospective Assessment Program [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage	13,100,000,000.0
Palo Verde Nuclear Generating Station [Member] | Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Maximum [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Liability insurance coverage	13,500,000,000.0
Liability insurance coverage sublimit	$ 2,250,000,000.00
Continuous Highwall Mining [Member] | San Juan Generating Station [Member]
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%
NMTRD Coal Severance Tax [Member] | Four Corners [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Assessed coal severance surtax, penalty and interest									$ 30,000,000
Share of settlement in addition to amounts previously paid		$ 100,000
Navajo Nation Allottee Matters [Member] | Public Service Company of New Mexico [Member]
Public Utilities, Commitments And Contingencies [Line Items]
Number of landowners claiming to be Navajo allottees (in landowners) | landowner						43				43
Number of allotment parcels' appraisal requested for review (in allotment parcels) | Allotment_Parcel							58
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel					10	6
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