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Regulatory and Rate Matters - PNM (Details)
 3 Months Ended 9 Months Ended 12 Months Ended 43 Months Ended
Sep. 30, 2016
USD ($)
MW
Sep. 29, 2016
USD ($)
Sep. 28, 2016
USD ($)
MW
Aug. 04, 2016
USD ($)
MW
Apr. 15, 2016
USD ($)
GWh
Feb. 03, 2016
Jan. 21, 2016
MW
Jan. 15, 2016
lease
MW
Aug. 27, 2015
USD ($)
Mar. 20, 2015
USD ($)
party
Jan. 15, 2015
Sep. 30, 2016
USD ($)
MW
Sep. 30, 2015
USD ($)
MW
Sep. 30, 2016
USD ($)
MW
Sep. 30, 2015
USD ($)
MW
Dec. 31, 2015
Sep. 30, 2016
USD ($)
MW
Jul. 08, 2016
MW
Apr. 26, 2016
turbine
MW
Feb. 26, 2016
USD ($)
Jan. 31, 2016
MW
Dec. 31, 2013
Mar. 01, 2013
Dec. 31, 2012
USD ($)
Public Utilities, General Disclosures [Line Items]                                                
Excess return on jurisdictional equity that would require refund 0.50%                     0.50%   0.50%     0.50%              
Public Service Company of New Mexico [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Duration of resource portfolio                           20 years                    
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 1 and 4 [Member] | Clean Air Act, Balanced Draft Technology [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Net book value $ 49,900,000                     $ 49,900,000   $ 49,900,000     $ 49,900,000              
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 1 and 4 [Member] | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] | Clean Air Act, Balanced Draft Technology [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Installation costs                                 52,300,000              
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 1 and 4 [Member] | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] | Clean Air Act, SNCR [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Installation costs                                 76,800,000              
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 2 and 3 [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Net book value 270,200,000                     270,200,000   270,200,000     270,200,000              
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 2 and 3 [Member] | Clean Air Act, SNCR [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Newly identified replacement gas-fired generation (in megawatts) | MW                         187   187       80          
Number of aeroderivative units (in turbines) | turbine                                     2          
Replacement gas-fired aeroderivative generation (in megawatts) | MW                                     40          
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station, Unit 2 Leases [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Number of megawatts purchased (in megawatts) | MW               64.1                         64.1      
Number of leases under which assets were purchased | lease               3                                
Estimated annual property tax expense $ 800,000                     $ 800,000   $ 800,000     $ 800,000              
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               114.6   114.6     114.6              
Net book value $ 76,900,000                     $ 76,900,000   $ 76,900,000     $ 76,900,000              
Net book value of capitalized improvements 41,700,000                     41,700,000   41,700,000     41,700,000              
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Estimated annual property tax expense 1,500,000                     1,500,000   1,500,000     1,500,000              
Estimated annual rent expense $ 18,100,000                     18,100,000   18,100,000     $ 18,100,000              
Number of leases under which lease term was extended | lease               4                                
Lease term extension period                     8 years                          
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Requested rate increase (decrease)                 $ 123,500,000                              
Requested rate increase (decrease) of non-fuel revenue                 $ 121,700,000         121,500,000                    
Requested return on equity                 10.50%                              
Requested rate increase (decrease) for fuel related costs                           (42,900,000)                    
Requested rate increase (decrease) for non-fuel related revenues                           $ (200,000)                    
Hearing examiner's recommended rate increase (decrease) of non-fuel revenue       $ 41,300,000                                        
Hearing examiner's recommended return on equity       9.575%                                        
Approved rate increase (decrease)     $ 61,200,000                                          
Estimated period of time for supreme court appeal decision 15 months                                              
Approved recovery of net operating loss carryforward                       2,100,000                        
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | San Juan Generating Station Units 1 and 4 [Member] | Clean Air Act, Balanced Draft Technology [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Requested base rate increase (decrease)                 $ 40,000,000                              
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | San Juan Generating Station [Member] | Refined Coal [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Hearing examiner's recommended percentage of revenue to be credited to customers       100.00%                                        
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Unit 2 Leases [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Hearing examiner's proposed disallowance of recovery       $ 163,300,000                                        
Initial rate base value     83,700,000                                          
Disallowance of the recovery underpreciated costs of capitalized leasehold improvements     43,800,000                                          
Pre-tax regulatory disallowance                       $ 6,800,000                        
Period of time for which capital improvements were disallowed 15 months                                              
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Hearing examiner's recommended rent expense recovery amount       0                                        
Hearing examiner's recommended recovery amount on conversions       0                                        
Recovery of assumed operating and maintenance expense savings annually     $ 300,000                                          
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Units 1 and 2 Leases [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Hearing examiner's recommended property tax recovery amount       0                                        
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Alvarado square [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Hearing examiner's recommended amount to not be recovered from retail customers       4,500,000                                        
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges       $ 4,500,000                                        
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Minimum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Profit incentive sliding scale multiplier   7.10%                                            
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Maximum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Reasonable cost threshold 3.00%                     3.00%   3.00%     3.00%         3.00%    
Profit incentive sliding scale multiplier   9.00%                                            
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Wind Energy [Member] | Minimum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of diversification 30.00%                     30.00%   30.00%     30.00%              
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Solar Energy [Member] | Minimum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of diversification 20.00%                     20.00%   20.00%     20.00%              
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Distributed Generation [Member] | Minimum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of diversification 3.00%                     3.00%   3.00%     3.00%              
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Renewable Technologies [Member] | Minimum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of diversification 5.00%                     5.00%   5.00%     5.00%              
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Required Percentage by 2011 [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of renewable energy in portfolio to electric sales 10.00%                     10.00%   10.00%     10.00%              
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Required Percentage by 2015 [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of renewable energy in portfolio to electric sales 15.00%                     15.00%   15.00%     15.00%              
Public Service Company of New Mexico [Member] | Renewable Portfolio Standard [Member] | Required Percentage by 2020 [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Required percentage of renewable energy in portfolio to electric sales 20.00%                     20.00%   20.00%     20.00%              
Public Service Company of New Mexico [Member] | Renewable Energy Rider [Member] | Maximum [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Proposal to collect funds under renewable energy procurement plan $ 42,400,000                     $ 42,400,000   $ 42,400,000     $ 42,400,000              
Annual revenue to be collected under rider rate $ 50,000,000                     50,000,000   $ 50,000,000     $ 50,000,000              
Approved return on equity           10.00%                   10.50%                
Public Service Company of New Mexico [Member] | 2017 Energy Efficiency and Load Management Program [Member] | Disincentives / Incentives Adder [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Program portfolio's total budget   $ 26,000,000     $ 28,000,000                                      
Incentive based on target savings         $ 2,400,000                                      
Targeted savings (in gigawatt hours) | GWh         75                                      
Public Service Company of New Mexico [Member] | Integrated Resource Plan, 2011 [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Frequency of IRP filings                           3 years                    
Planning period covered of IRP                           20 years                    
Public Service Company of New Mexico [Member] | Advanced Metering Infrastructure Costs [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Estimated costs to be recovered                                       $ 87,200,000        
Estimated future investment                                       $ 33,000,000        
Public Service Company of New Mexico [Member] | Formula Transmission Rate Case [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Approved rate increase (decrease)                   $ 1,300,000                            
Amount of regulatory costs not yet approved                                               $ 3,200,000
Return on equity                   10.00%                           10.81%
Percentage ownership of EIP transmission line                                             60.00%  
Number of other parties entered into settlement agreement | party                   5                            
Public Service Company of New Mexico [Member] | Firm Requirements Wholesale Power Rate Case, Navopache [Member] [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Number of megawatts served under a short-term coordination tariff | MW             10                                  
Revenue for power sold under PSA                       $ 4,800,000 $ 6,300,000 $ 14,800,000 $ 19,700,000                  
PNMR Development [Member]                                                
Public Utilities, General Disclosures [Line Items]                                                
Solar generation capacity (in megawatts) | MW                                   30            
Construction of solar generation capacity (in megawatts) | MW 10                     10   10     10