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Fair Value of Derivative and Other Financial Instruments - Available for Sale Securities (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 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 (2,600,000) $ (100,000) (3,800,000) $ 1,000,000    
Proceeds from sales 167,359,000 91,657,000 794,088,000 358,045,000    
Gross realized gains 7,549,000 7,971,000 13,570,000 16,617,000    
Gross realized (losses) (6,192,000) $ (2,236,000) (10,869,000) $ (5,321,000)    
Nuclear Decommissioning Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale securities 292,800,000   292,800,000     $ 293,700,000
Mine Reclamation Trust | Recurring | PNM            
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale securities $ 30,300,000   $ 30,300,000     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