Regulatory and Rate Matters - PNM (Details) $ in Thousands																											1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended		30 Months Ended
	Sep. 30, 2018 USD ($) lease service_rate MW	Aug. 24, 2018 power_purchase_agreement Facility MW	Jun. 30, 2018 USD ($)	Apr. 13, 2018 USD ($)	Mar. 17, 2018 USD ($)	Jan. 31, 2018 USD ($)	Jan. 16, 2018 USD ($)	Jan. 10, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 08, 2017 USD ($)	Jul. 26, 2017 USD ($) GWh	Jul. 03, 2017	Jun. 21, 2017	Jun. 01, 2017 GWh MW	May 23, 2017 USD ($)	Apr. 14, 2017 USD ($) GWh program	Jan. 11, 2017 USD ($)	Dec. 07, 2016 USD ($)	Sep. 28, 2016 USD ($) MW	Aug. 04, 2016 USD ($) MW	May 04, 2016	Apr. 15, 2016 USD ($) GWh program	Jan. 16, 2016 lease	Aug. 27, 2015 USD ($)	Jan. 15, 2015	Jul. 01, 2014	Sep. 30, 2016 USD ($)	Sep. 30, 2018 USD ($) lease service_rate MW	Dec. 31, 2017 USD ($) power_purchase_agreement	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) lease service_rate MW	Sep. 30, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Aug. 22, 2018 USD ($)	Jun. 20, 2018 MW	Mar. 21, 2018 MW	Nov. 07, 2017	Oct. 31, 2017 USD ($)	Oct. 17, 2017 MW	Sep. 05, 2017 MW	May 12, 2017 USD ($) signatory	Oct. 26, 2016 MW	Jul. 01, 2016 USD ($)	Jun. 30, 2016 meeting	Feb. 26, 2016 USD ($)	Jan. 31, 2016 lease MW	Jan. 15, 2016 USD ($) lease MW
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs																												$ (1,645)		$ 0	$ 149	$ 0
Increase (decrease) in Non-fuel Revenue Requirement						$ 10,300
Percent of non-fuel revenue requirement change implemented						50.00%
Excess return on jurisdictional equity that would require refund	0.50%																											0.50%			0.50%
Application filed for new electric service rates | service_rate	2																											2			2
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs																												$ (1,645)		0	$ 149	0
Filing frequency for energy efficiency plans														3 years
Modification percentage to funding levels													0.1
Planning period covered of IRP												20 years
Action plan, covered period												4 years
Number of statewide meetings hosted | meeting																																														17
Estimated insurance deductible					$ 2,000
Exposure to cost of repairs					$ 1,000
Ownership percentage					50.00%
PNMR Development
Public Utilities, Commitments And Contingencies [Line Items]
Solar generation capacity (in megawatts) | MW	30																											30			30											50
Construction of solar generation capacity (in megawatts) | MW	10																											10			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	$ 147,500																											$ 147,500			$ 147,500
2014 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Future test year period																		12 months
Period of time after the filing of a rate case application																		13 months
Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts of Solar PV facilities | MW	157																											157			157
Current output in the geothermal facility (in megawatts) | MW	4																											4			4
Solar generation capacity (in megawatts) | MW	95.9																											95.9			95.9
Requested approval to procure a new solar facilities to be constructed (in megawatts) | MW	50																											50			50
Renewable Energy Rider | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Recorded revenues from renewable rider																												$ 8,700		$ 9,400	$ 30,400	$ 33,600
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]
Public Utilities, Initial Capital Investments To Be Recovered																																				$ 20,900
Public Utilities, Other Expenses To Be Recovered																																				$ 7,400
Advanced Metering Infrastructure Costs | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Estimated costs to be recovered																																											$ 95,100				$ 87,200
Estimated future investment																																															$ 33,000
Facebook Data Center
Public Utilities, Commitments And Contingencies [Line Items]
Number of additional PPAs | power_purchase_agreement		2																											3
Power purchase agreement term		25 years																											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																	64.1	64.1
Number of leases under which assets were purchased | lease	3																						3					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	$ 73,900																											$ 73,900			$ 73,900
Net book value of capitalized improvements	38,300																											38,300			38,300
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 underpreciated costs of capitalized leasehold improvements																			43,800
Period of time for which capital improvements were disallowed																											15 months
Pre-tax regulatory disallowance	900		$ 1,800						$ 3,100																		$ 6,800
Period of time to issue a decision and take action on remanded issues			5 months						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
Palo Verde Nuclear Generating Station, Unit 1 Leases, extended | 2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Recovery of assumed operating and maintenance expense savings annually																			$ 300
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	$ 50,300																											$ 50,300			$ 50,300
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%
Number of additional signatories | signatory																																											13
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate															21 years
Regulatory disallowance									$ 27,900																				$ 27,900
Action plan, covered period																															4 years
NMPRC | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)							$ (4,400)	$ (9,100)							$ 62,300
Requested return on equity															9.575%
Requested initial rate increase (decrease)															$ 32,300
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate															3 years
Requested rate increase (decrease) duplicative amount							$ 4,700
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
Action plan, covered period																										4 years
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%																											10.00%			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%																											15.00%			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%																											20.00%			20.00%
Minimum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Profit incentive sliding scale multiplier																	7.10%
Minimum | Wind | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of diversification	30.00%																											30.00%			30.00%
Minimum | Solar | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of diversification	20.00%																											20.00%			20.00%
Minimum | Distributed generation | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of diversification	3.00%																											3.00%			3.00%
Minimum | Other | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of diversification	5.00%																											5.00%			5.00%
Maximum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Reasonable cost threshold	3.00%																3.00%											3.00%			3.00%
Profit incentive sliding scale multiplier																	9.00%
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
Disincentives/Incentives Adder | 2017 Energy Efficiency and Load Management Program | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of programs in the proposed program portfolio | program																						10
Program portfolio's total budget																	$ 26,000					$ 28,000
Incentive based on target savings																						$ 2,400
Targeted savings (in gigawatt hours) | GWh																						75
Profit incentive																	$ 1,800
Disincentives/Incentives Adder | Proposed 2018 Portfolio | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of programs in the proposed program portfolio | program																10
Program portfolio's total budget										$ 23,600						$ 25,100
Incentive based on target savings																$ 1,900
Targeted savings (in gigawatt hours) | GWh																53
Projected incentive earnings				$ 2,300							$ 1,900					$ 2,100
Targeted savings (in gigawatt hours) | GWh											69					70
Disincentives/Incentives Adder | Proposed 2019 Portfolio | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of programs in the proposed program portfolio | program																10
Program portfolio's total budget										$ 24,900						$ 28,200
Incentive based on target savings											$ 1,700					$ 2,100
Four Corners
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowance																																								$ 90,100					$ 148,100
Regulatory disallowances and restructuring costs								$ 47,600
Forecast
Public Utilities, Commitments And Contingencies [Line Items]
Pre-tax regulatory disallowance																																		$ 36,800	$ 58,000
Forecast | NMPRC | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Proposed revision to rider that will allow for recovery																																	$ 49,600	$ 43,500
Discount Rate
Public Utilities, Commitments And Contingencies [Line Items]
WACC										0.0959																													0.0771