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Stock-Based Compensation - Performance Equity Plan and Accounting for Stock Awards (Details) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | May 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense for stock-based arrangements | $ 8.1 | $ 6.4 | $ 7.1 |
Restricted Shares and Performance-Based Shares |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Unrecognized compensation expense | $ 4.0 |  |  |
Period to recognize compensation expense | 1 year 6 months 7 days |  |  |
Public Service Company of New Mexico |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense for stock-based arrangements | $ 5.5 | 4.2 | 4.9 |
Texas-New Mexico Power Company |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense for stock-based arrangements | $ 2.6 | $ 2.2 | $ 2.2 |
Performance Equity Plan |  |  |  |
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
Charge to share pool for each share awarded | 5 |  |  |
Performance Equity Plan | Nonemployee Members of the Board of Directors |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Vesting period for awards | 1 year |  |  |

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