Commitments and Contingencies - Coal Supply (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Aug. 01, 2016 payment	Jan. 31, 2016 USD ($)
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets	$ 49,259	$ 54,808
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets	39,203	43,516
PNM | Loss on long-term purchase commitment | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual	70,000	70,100
Final reclamation, capped amount to be collected		100,000
PNM | Loss on long-term purchase commitment | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Loss contingency accrual	24,300	23,200
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss	94,400
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Estimate of possible loss	40,000
Coal supply | PNM | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Other current assets	26,300
Coal supply | NM Capital | San Juan Generating Station
Public Utilities, Commitments And Contingencies [Line Items]
Requirement to post reclamation bonds	118,700
Cash used to support bank letter or credit arrangement	30,300				$ 30,300
Increase in coal mine decommissioning liability | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded	300
Increase in coal mine decommissioning liability | PNM | Underground and Surface
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded		39,200
Increase in coal mine decommissioning liability | PNM | Surface
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded		29,800
Increase in coal mine decommissioning liability | PNM | Underground
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance, additional amount recorded		9,400
Increase in coal mine decommissioning liability | PNM | Loss on long-term purchase commitment
Public Utilities, Commitments And Contingencies [Line Items]
Estimated underpaid surface mining royalties under proposed rate change			$ 2,500
Mine Reclamation Trust | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required contribution to Reclamation Trust, current fiscal year	6,100
Reclamation trust funding, year 2	10,200
Reclamation trust funding, year 3	10,900
San Juan Generating Station | Loss on long-term purchase commitment | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Annual funding post-term reclamation trust		10,000
Four Corners | Mine Reclamation Trust | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required contribution to Reclamation Trust, current fiscal year		$ 2,300
Reclamation trust funding, year 2	$ 2,300
Number of annual installment payments | payment				13