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Fair Value of Derivative and Other Financial Instruments - Items Recorded and Presented by Level of Hierarchy (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Transfers between levels $ 0 $ 0
Long-term debt 3,023,159,000 2,703,810,000
Other investments 299,000 297,000
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 0 0
Other investments 299,000 297,000
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 3,023,159,000 2,703,810,000
Other investments 0 0
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 0 0
Other investments 0 0
Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 2,944,241,000 2,670,111,000
Other investments 299,000 297,000
PNM    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 1,744,864,000 1,668,736,000
Other investments 93,000 91,000
PNM | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 0 0
Other investments 93,000 91,000
PNM | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 1,744,864,000 1,668,736,000
Other investments 0 0
PNM | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 0 0
Other investments 0 0
PNM | Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 1,706,873,000 1,656,490,000
Other investments 93,000 91,000
TNMP    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 838,705,000 597,236,000
Other investments 206,000 206,000
TNMP | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 0 0
Other investments 206,000 206,000
TNMP | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 838,705,000 597,236,000
Other investments 0 0
TNMP | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 0 0
Other investments 0 0
TNMP | Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt 798,941,000 575,398,000
Other investments 206,000 206,000
Recurring | PNM    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 351,563,000 328,242,000
Recurring | PNM | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 152,868,000 136,150,000
Recurring | PNM | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 196,608,000 189,936,000
Recurring | PNM | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 2,087,000 2,156,000
Investments, unrealized gain 9,002,000 2,600,000
Recurring | PNM | Commodity derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets 3,351,000 3,594,000
Commodity derivative liabilities (3,415,000) (3,688,000)
Net (64,000) (94,000)
Recurring | PNM | Commodity derivatives | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets 0 0
Commodity derivative liabilities 0 0
Net 0 0
Recurring | PNM | Commodity derivatives | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets 3,351,000 3,594,000
Commodity derivative liabilities (3,415,000) (3,688,000)
Net (64,000) (94,000)
Recurring | PNM | Commodity derivatives | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commodity derivative assets 0 0
Commodity derivative liabilities 0 0
Net 0 0
Recurring | PNM | Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 10,339,000 11,472,000
Recurring | PNM | Cash and cash equivalents | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 10,339,000 11,472,000
Recurring | PNM | Cash and cash equivalents | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Recurring | PNM | Cash and cash equivalents | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Recurring | PNM | Corporate stocks, common    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 36,025,000 32,997,000
Recurring | PNM | Corporate stocks, common | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 36,025,000 32,997,000
Recurring | PNM | Corporate stocks, common | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Recurring | PNM | Corporate stocks, common | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Recurring | PNM | Corporate stocks, preferred    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 8,138,000 7,258,000
Recurring | PNM | Corporate stocks, preferred | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 1,854,000 1,654,000
Recurring | PNM | Corporate stocks, preferred | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 6,284,000 5,604,000
Recurring | PNM | Corporate stocks, preferred | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Recurring | PNM | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 78,699,000 70,777,000
Recurring | PNM | Mutual funds and other | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 78,688,000 70,777,000
Recurring | PNM | Mutual funds and other | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 11,000 0
Recurring | PNM | Mutual funds and other | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Recurring | PNM | Corporate stocks, common | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, unrealized gain  
Recurring | PNM | Corporate stocks, preferred | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, unrealized gain  
Recurring | PNM | Mutual funds and other | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, unrealized gain  
Recurring | PNM | U.S. Government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 36,153,000 29,503,000
Recurring | PNM | U.S. Government | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 25,074,000 18,662,000
Recurring | PNM | U.S. Government | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 11,079,000 10,841,000
Recurring | PNM | U.S. Government | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 0 0
Investments, unrealized gain 1,489,000 1,098,000
Recurring | PNM | International Government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 11,577,000 8,435,000
Recurring | PNM | International Government | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 46,000 0
Recurring | PNM | International Government | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 11,531,000 8,435,000
Recurring | PNM | International Government | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 0 0
Investments, unrealized gain 345,000 90,000
Recurring | PNM | Municipals    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 49,160,000 53,642,000
Recurring | PNM | Municipals | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 0 0
Recurring | PNM | Municipals | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 49,160,000 53,642,000
Recurring | PNM | Municipals | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 0 0
Investments, unrealized gain 1,337,000 489,000
Recurring | PNM | Corporate and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 121,472,000 114,158,000
Recurring | PNM | Corporate and other | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 842,000 588,000
Recurring | PNM | Corporate and other | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 118,543,000 111,414,000
Recurring | PNM | Corporate and other | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale debt securities 2,087,000 2,156,000
Investments, unrealized gain $ 5,831,000 $ 923,000