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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, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Jan. 01, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale debt securities, fair value $ 0   $ 0      
Other than temporary impairments     0      
PNM            
Debt Securities, Available-for-sale [Line Items]            
(Increase)/decrease in other than temporary losses of available-for-sale securities, net portion recognized in earnings (800,000) $ 100,000 (4,600,000) $ 1,100,000    
Proceeds from sales 117,801,000 98,532,000 911,899,000 456,577,000    
Gross realized gains 3,460,000 8,128,000 17,030,000 24,745,000    
Gross realized (losses) (3,149,000) $ (2,829,000) (14,018,000) $ (8,150,000)    
Nuclear Decommissioning Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale securities 298,400,000   298,400,000     $ 293,700,000
Mine Reclamation Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale securities $ 33,300,000   $ 33,300,000     29,800,000
Retained Earnings            
Debt Securities, Available-for-sale [Line Items]            
Cumulative effect adjustment (Note 7)           11,208,000
Retained Earnings | PNM            
Debt Securities, Available-for-sale [Line Items]            
Cumulative effect adjustment (Note 7)           $ 11,208,000
Accounting Standards Update 2016-01 | Retained Earnings            
Debt Securities, Available-for-sale [Line Items]            
Cumulative effect adjustment (Note 7)         $ 11,200,000