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Regulatory and Rate Matters - PNM (Details)
$ in Millions
 1 Months Ended 3 Months Ended 12 Months Ended 49 Months Ended
Apr. 14, 2017
USD ($)
GWh
program
Mar. 27, 2017
Feb. 28, 2017
USD ($)
Jan. 11, 2017
USD ($)
Dec. 07, 2016
USD ($)
Nov. 30, 2016
Sep. 28, 2016
USD ($)
MW
Aug. 04, 2016
USD ($)
MW
Jun. 01, 2016
MW
May 04, 2016
Apr. 15, 2016
USD ($)
GWh
program
Jan. 15, 2016
USD ($)
lease
MW
Nov. 30, 2015
Aug. 27, 2015
USD ($)
Jan. 15, 2015
Jul. 01, 2014
Sep. 30, 2016
USD ($)
Mar. 31, 2017
USD ($)
MWh
lease
MW
Mar. 31, 2016
USD ($)
Dec. 31, 2017
portfolio
Dec. 31, 2016
USD ($)
MWh
Dec. 31, 2015
MW
Mar. 31, 2017
USD ($)
lease
MW
Oct. 26, 2016
MW
Jul. 08, 2016
service_rate
MW
Apr. 26, 2016
MW
Feb. 26, 2016
USD ($)
Jan. 31, 2016
lease
MW
Public Utilities, General Disclosures [Line Items]                                                        
Excess return on jurisdictional equity that would require refund                                   0.50%         0.50%          
Application filed for new electric service rates | service_rate                                                 2      
NMPRC                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Requested rate increase (decrease)         $ 99.2                                              
Requested return on equity         10.125%                                              
Unopposed motion filed for extension of procedural schedule   1 month                                                    
Proposed revision to rider that will allow for recovery     $ 42.7                                                  
Action plan, covered period                                   4 years                    
Public Service Company of New Mexico | Tri-State                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Term of hazard sharing agreement                 1 year                                      
Each party will sell the other party capacity (in megawatts) | MW                 100                                      
Number of megawatt hours sold | MWh                                   213,533     482,342              
Megawatt hours sold                                   $ 4.8     $ 12.8              
Number of megawatt hours purchased | MWh                                   205,156     484,632              
Megawatt hours purchased                                   $ 4.6     $ 12.9              
Public Service Company of New Mexico | Forecast                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Planning period covered of IRP                                       20 years                
Action plan, covered period                                       4 years                
Number of resource portfolios | portfolio                                       2                
Public Service Company of New Mexico | NMPRC                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Action plan, covered period                               4 years                        
Notice of Proposed Dismissal, period to show good cause                   30 days                                    
Public Service Company of New Mexico | NMPRC | Tri-State                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Term of hazard sharing agreement           5 years                       5 years                    
Public Service Company of New Mexico | San Juan Generating Station Units 1 and 4 | Clean Air Act, Balanced Draft Technology                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Net book value                                   $ 50.0         $ 50.0          
Public Service Company of New Mexico | San Juan Generating Station Units 1 and 4 | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other Costs | Clean Air Act, Balanced Draft Technology                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Installation costs               $ 52.3                                        
Public Service Company of New Mexico | San Juan Generating Station Units 1 and 4 | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other Costs | Clean Air Act, SNCR                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Installation costs                                             77.9          
Public Service Company of New Mexico | San Juan Generating Station | Forecast                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of portfolios for continuing operations | portfolio                                       1                
Number of portfolios with discontinued operations | portfolio                                       1                
Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Net book value                                   $ 265.4         $ 265.4          
Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3 | Clean Air Act, SNCR                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Newly identified replacement gas-fired generation (in megawatts) | MW                                                   80    
Replacement gas-fired aeroderivative generation (in megawatts) | MW                                                   40    
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 2 Leases                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of megawatts purchased (in megawatts) | MW                       64.1           64.1         64.1         64.1
Number of leases under which assets were purchased | lease                       3           3         3         3
Estimated annual property tax expense                       $ 0.8                                
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                                   $ 76.9         $ 76.9          
Net book value of capitalized improvements                                   39.9         39.9          
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Estimated annual property tax expense                       $ 1.5                                
Estimated annual rent expense               $ 18.1                                        
Number of leases under which lease term was extended | lease                       4                                
Lease term extension period                             8 years                          
Public Service Company of New Mexico | 2015 Electric Rate Case                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Requested rate increase (decrease)                           $ 123.5                            
Requested rate increase (decrease) of non-fuel revenue               121.5           $ 121.7       121.5                    
Requested return on equity                           10.50%                            
Requested rate increase (decrease) for fuel related costs                                   (42.9)                    
Requested rate increase (decrease) for non-fuel related revenues                                   (0.2)                    
Hearing examiner's recommended rate increase (decrease) of non-fuel revenue               $ 41.3                                        
Hearing examiner's recommended return on equity               9.575%                                        
Approved rate increase (decrease)             $ 61.2                                          
Estimated period of time for supreme court appeal decision                                 15 months                      
Approved recovery of net operating loss carryforward                                 $ 2.1                      
Estimate of possible loss                                   $ 154.9         $ 154.9          
Public Service Company of New Mexico | 2015 Electric Rate Case | San Juan Generating Station Units 1 and 4 | Clean Air Act, Balanced Draft Technology                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Requested base rate increase (decrease)               $ 40.0                                        
Public Service Company of New Mexico | 2015 Electric Rate Case | San Juan Generating Station | Refined coal                                                        
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 | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 2 Leases                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Hearing examiner's proposed disallowance of recovery               $ 163.3                                        
Initial rate base value             83.7                                          
Disallowance of the recovery underpreciated costs of capitalized leasehold improvements             43.8                                          
Period of time for which capital improvements were disallowed                                 15 months                      
Pre-tax regulatory disallowance                                 $ 6.8                      
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Recovery of assumed operating and maintenance expense savings annually             $ 0.3                                          
Public Service Company of New Mexico | 2015 Electric Rate Case | Alvarado square                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Hearing examiner's recommended amount to not be recovered from retail customers               $ 4.5                                        
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges                                 $ 4.5                      
Public Service Company of New Mexico | 2014 Electric Rate Case                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Period of time after the filing of a rate case application                         13 months                              
Public Service Company of New Mexico | Renewable Portfolio Standard                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of megawatts of Solar PV facilities | MW                                   107       40 107          
Current output in the geothermal facility (in megawatts) | MW                                   4         4          
Solar generation capacity (in megawatts) | MW                                   66.8         66.8          
Public Service Company of New Mexico | Renewable Portfolio Standard | Minimum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Profit incentive sliding scale multiplier       7.10%                                                
Public Service Company of New Mexico | Renewable Portfolio Standard | Maximum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Reasonable cost threshold       3.00%                           3.00%         3.00%          
Profit incentive sliding scale multiplier       9.00%                                                
Public Service Company of New Mexico | Renewable Portfolio Standard | Wind | Minimum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of diversification                                   30.00%         30.00%          
Public Service Company of New Mexico | Renewable Portfolio Standard | Solar | Minimum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of diversification                                   20.00%         20.00%          
Public Service Company of New Mexico | Renewable Portfolio Standard | Distributed generation | Minimum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of diversification                                   3.00%         3.00%          
Public Service Company of New Mexico | Renewable Portfolio Standard | Other | Minimum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of diversification                                   5.00%         5.00%          
Public Service Company of New Mexico | Renewable Portfolio Standard | Required Percentage by 2011                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of renewable energy in portfolio to electric sales                                   10.00%         10.00%          
Public Service Company of New Mexico | Renewable Portfolio Standard | Required Percentage by 2015                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of renewable energy in portfolio to electric sales                                   15.00%         15.00%          
Public Service Company of New Mexico | Renewable Portfolio Standard | Required Percentage by 2020                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Required percentage of renewable energy in portfolio to electric sales                                   20.00%         20.00%          
Public Service Company of New Mexico | Proposed 2018 Portfolio | Disincentives/Incentives Adder | Subsequent event                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of programs in the proposed program portfolio | program 10                                                      
Program portfolio's total budget $ 25.1                                                      
Incentive based on target savings $ 1.9                                                      
Targeted savings (in gigawatt hours) | GWh 53                                                      
Public Utilities, Energy Efficiency and Load Management Programs, Projected Earned Incentive $ 2.1                                                      
Public Utilities, Energy Efficiency and Load Management Programs, Targeted Energy Savings | GWh 70                                                      
Public Service Company of New Mexico | Proposed 2019 Portfolio | Disincentives/Incentives Adder | Subsequent event                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of programs in the proposed program portfolio | program 10                                                      
Program portfolio's total budget $ 28.2                                                      
Incentive based on target savings $ 2.1                                                      
Public Service Company of New Mexico | Renewable Portfolio Standard Supplemental Procurement                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of megawatts of Solar PV facilities | MW                                   1.5         1.5          
Public Service Company of New Mexico | Renewable Portfolio Standard 2014                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of megawatts for wind energy | MW                                   204       102            
Public Service Company of New Mexico | Renewable Energy Rider                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Recorded revenues from renewable rider                                   $ 11.0 $ 8.4                  
Public Service Company of New Mexico | Renewable Energy Rider | Maximum                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Proposal to collect funds under renewable energy procurement plan                                         $ 42.4              
Annual revenue to be collected under rider rate                                   $ 50.0         $ 50.0          
Public Service Company of New Mexico | 2017 Energy Efficiency and Load Management Program | Disincentives/Incentives Adder                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of programs in the proposed program portfolio | program                     10                                  
Program portfolio's total budget       $ 26.0             $ 28.0                                  
Incentive based on target savings                     $ 2.4                                  
Targeted savings (in gigawatt hours) | GWh                     75                                  
Profit incentive       $ 1.8                                                
Public Service Company of New Mexico | Integrated Resource Plan, 2011                                                        
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 | Advanced Metering Infrastructure Costs                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Estimated costs to be recovered                                                     $ 87.2  
Estimated future investment                                                     $ 33.0  
Public Service Company of New Mexico | Firm Requirements Wholesale Power Rate Case, Navopache                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Number of megawatts served under a short-term coordination tariff | MW                                   10                    
Revenue for power sold under PSA                                   $ 1.1 $ 5.4                  
PNMR Development                                                        
Public Utilities, General Disclosures [Line Items]                                                        
Solar generation capacity (in megawatts) | MW                                                 30      
Construction of solar generation capacity (in megawatts) | MW                                                 10