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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale securities for which carrying value exceeds fair value $ 0   $ 0    
Held-to-maturity securities for which carrying value exceeds fair value 0   0    
Impairments considered to be other than temporary     0    
PNM          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 306,444,000   306,444,000   $ 272,977,000
Unrealized Gains 18,631,000   18,631,000   7,449,000
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings 100,000 $ 100,000 1,100,000 $ 1,000,000  
Proceeds from sales 98,532,000 86,975,000 456,577,000 280,989,000  
Gross realized gains 8,128,000 7,026,000 24,745,000 27,273,000  
Gross realized (losses) (2,829,000) $ (2,565,000) (8,150,000) $ (12,913,000)  
PNM | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 306,444,000   306,444,000   272,977,000
PNM | Nuclear Decommissioning Trust | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 283,000,000   283,000,000   253,900,000
PNM | Mine Reclamation Trust | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 23,400,000   23,400,000   19,100,000
PNM | Cash and cash equivalents          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 8,151,000   8,151,000   23,683,000
Unrealized Gains 0   0   0
PNM | Cash and cash equivalents | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 8,151,000   8,151,000   23,683,000
PNM | Domestic value          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 72,162,000   72,162,000   34,796,000
Unrealized Gains 5,252,000   5,252,000   1,135,000
PNM | Domestic value | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 72,162,000   72,162,000   34,796,000
PNM | Domestic growth          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 73,345,000   73,345,000   47,595,000
Unrealized Gains 5,775,000   5,775,000   3,032,000
PNM | Domestic growth | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 73,345,000   73,345,000   47,595,000
PNM | International and other          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 43,167,000   43,167,000   27,481,000
Unrealized Gains 4,865,000   4,865,000   2,029,000
PNM | International and other | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 43,167,000   43,167,000   27,481,000
PNM | U.S. Government          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 28,960,000   28,960,000   40,962,000
Unrealized Gains 307,000   307,000   115,000
PNM | U.S. Government | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 28,960,000   28,960,000   40,962,000
PNM | Municipals          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 41,131,000   41,131,000   43,789,000
Unrealized Gains 998,000   998,000   585,000
PNM | Municipals | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 41,131,000   41,131,000   43,789,000
PNM | Corporate and other          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value 39,528,000   39,528,000   54,671,000
Unrealized Gains 1,434,000   1,434,000   553,000
PNM | Corporate and other | Fair Value, Measurements, Recurring          
Schedule of Available-for-sale Securities [Line Items]          
Fair Value $ 39,528,000   $ 39,528,000   $ 54,671,000