Regulatory and Rate Matters - PNM (Details) $ in Thousands															1 Months Ended	3 Months Ended			12 Months Ended	30 Months Ended
	Aug. 24, 2018 Facility power_purchase_agreement MW	Jun. 30, 2018 USD ($)	Jan. 31, 2018	Jan. 10, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jul. 03, 2017	May 23, 2017 USD ($)	Dec. 07, 2016 USD ($)	Sep. 28, 2016 USD ($) MW	Aug. 04, 2016 USD ($) MW	May 04, 2016	Jan. 16, 2016 lease	Aug. 27, 2015 USD ($)	Jan. 15, 2015	Sep. 30, 2016 USD ($)	Mar. 31, 2019 USD ($) lease MW	Mar. 31, 2018 USD ($)	Dec. 31, 2017	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Mar. 22, 2019	Aug. 22, 2018 USD ($)	Jun. 20, 2018 MW	Mar. 21, 2018 MW	Oct. 31, 2017 USD ($)	Oct. 17, 2017 MW	Sep. 05, 2017 MW	Jun. 01, 2017 GWh MW	Oct. 26, 2016 MW	Jul. 01, 2016 USD ($)	Jan. 31, 2016 lease MW	Jan. 15, 2016 USD ($) lease MW
Public Utilities, Commitments And Contingencies [Line Items]
Pre-tax regulatory disallowance																				$ 58,000
Regulatory disallowances and restructuring costs																$ 1,345	$ 0
Percent of non-fuel revenue requirement change implemented			50.00%
Ownership percentage																50.00%
Excess return on jurisdictional equity that would require refund																0.50%
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs																$ 1,345	0
Planning period covered of IRP						20 years
Action plan, covered period						4 years
PNMR Development
Public Utilities, Commitments And Contingencies [Line Items]
Solar generation capacity (in megawatts) | MW																30											50
Ownership percentage																50.00%
Construction of solar generation capacity (in megawatts) | MW																10
2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)													$ 123,500
Requested rate increase (decrease) of non-fuel revenue										$ 121,500			$ 121,700			$ 121,500
Requested return on equity													10.50%
Requested rate increase (decrease) for fuel related costs																42,900
Requested rate increase (decrease) for non-fuel related revenues																200
Hearing examiner's recommended rate increase (decrease) of non-fuel revenue										$ 41,300
Hearing examiner's recommended return on equity										9.575%
Approved rate increase (decrease)									$ 61,200
Estimated period of time for supreme court appeal decision															15 months
Estimate of possible loss																$ 144,700
Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts of Solar PV facilities | MW																157
Current output in the geothermal facility (in megawatts) | MW																15
Solar generation capacity (in megawatts) | MW																106.8
Requested approval to procure a new solar facilities to be constructed (in megawatts) | MW																50
Renewable Energy Rider | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Recorded revenues from renewable rider																$ 12,700	$ 10,900
Integrated Resource Plan, 2011 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Frequency of IRP filings																3 years
Planning period covered of IRP																20 years
Energy Imbalance Market | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Initial capital investments to be recovered																						$ 20,900
Other expenses to be recovered																						$ 7,400
Facebook Data Center
Public Utilities, Commitments And Contingencies [Line Items]
Number of additional PPAs | power_purchase_agreement	2
Power purchase agreement term																		25 years
Facebook Data Center | Casa Mesa Wind
Public Utilities, Commitments And Contingencies [Line Items]
Solar generation capacity (in megawatts) | MW																								50
Facebook Data Center | Avangrid Renewables, LLC
Public Utilities, Commitments And Contingencies [Line Items]
Solar generation capacity (in megawatts) | MW																								166
Facebook Data Center | Route 66 Solar Energy Center
Public Utilities, Commitments And Contingencies [Line Items]
Solar generation capacity (in megawatts) | MW	100																							50
Number of solar facilities | Facility	2
Palo Verde Nuclear Generating Station, Unit 2 Leases | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts purchased (in megawatts) | MW																64.1															64.1	64.1
Number of leases under which assets were purchased | lease												3				3															3	3
Estimated annual property tax expense																																$ 800
Number of leases under which lease term was extended | lease												1
Lease term extension period												8 years
Number of megawatts nuclear generation (in megawatts) | MW									114.6	114.6																			114.6
Net book value																$ 72,600
Net book value of capitalized improvements																37,600
Palo Verde Nuclear Generating Station, Unit 2 Leases | 2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Hearing examiner's proposed disallowance of recovery										$ 163,300
Initial rate base value									$ 83,700
Disallowance of the recovery undepreciated costs of capitalized leasehold improvements									$ 43,800
Period of time for which capital improvements were disallowed															15 months
Pre-tax regulatory disallowance		$ 4,000			$ 3,100										$ 6,800	1,300
Period of time to issue a decision and take action on remanded issues					7 months
Alvarado square | 2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Hearing examiner's recommended amount to not be recovered from retail customers										4,500
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges															$ 4,500
Palo Verde Nuclear Generating Station, Unit 1 Leases, extended | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Estimated annual property tax expense																																$ 1,500
Estimated annual rent expense										18,100
Number of leases under which lease term was extended | lease																																4
Lease term extension period														8 years
Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Net book value																$ 49,700
Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 | 2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested base rate increase (decrease)										40,000
Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other Costs | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Installation capital costs										$ 52,300
NMPRC
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)								$ 99,200
Requested return on equity								10.125%
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate							21 years
Action plan, covered period																4 years
NMPRC | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)							$ 10,300
Requested return on equity							9.575%
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate							3 years
Requested approval to procure additional gigawatt hours in year 1 | GWh																												80
Requested approval to procure a new solar facilities to be constructed (in megawatts) | MW																							50			50		50
Proposed revision to rider that will allow for recovery																			$ 49,600
Notice of proposed dismissal, period to show good cause											30 days
Required Percentage by 2011 | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of renewable energy in portfolio to electric sales																10.00%
Required Percentage by 2015 | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of renewable energy in portfolio to electric sales																15.00%
Required Percentage by 2020 | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of renewable energy in portfolio to electric sales																20.00%
Required Percentage by 2020 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, Commitments And Contingencies [Line Items]
Renewable energy, percentage																					0.20
Required Percentage by 2025 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, Commitments And Contingencies [Line Items]
Renewable energy, percentage																					0.40
Required Percentage by 2030 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, Commitments And Contingencies [Line Items]
Renewable energy, percentage																					0.50
Required Percentage by 2040 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, Commitments And Contingencies [Line Items]
Renewable energy, percentage																					0.80
Required Percentage by 2045 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, Commitments And Contingencies [Line Items]
Renewable energy, percentage																					1.00
Maximum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Reasonable cost threshold																3.00%
New Mexico Wind | Renewable Portfolio Standard 2014 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts for wind energy | MW																204
New Mexico Wind | NMPRC | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested approval to procure additional gigawatt hours in year 2 | GWh																												105
Red Mesa Wind | Renewable Portfolio Standard 2014 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts for wind energy | MW																102
Lightning Dock Geothermal | NMPRC | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested approval to procure additional gigawatt hours in year 1 | GWh																												55
Requested approval to procure additional gigawatt hours in year 2 | GWh																												77
Four Corners
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs				$ 47,600
Regulatory disallowance																									$ 90,100					$ 148,100