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Stock-Based Compensation Plans Performance Equity Plan/ Accounting for Stock Awards and ESPP (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expense for stock-based arrangements $ 4.9 $ 5.9 $ 5.3
Public Service Company of New Mexico [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expense for stock-based arrangements 3.6 4.2 3.8
Texas-New Mexico Power Company [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expense for stock-based arrangements 1.3 $ 1.7 $ 1.5
Restricted Stock and Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation expense $ 5.7    
Period to recognize compensation expense 1 year 5 months    
Performance Equity Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period for awards 3 years    
Vesting rate 100.00%    
Number of shares authorized (in shares) 13,500,000.0