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Stock-Based Compensation - Performance Equity Plan and Accounting for Stock Awards (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018 | Jan. 01, 2017 | May 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Compensation expense for stock-based arrangements | $ 6,400 | $ 7,100 | $ 6,200 |  |  |
Cumulative effect adjustment |  |  |  | $ 0 | $ 10,382 |
Accumulated deferred income taxes | (626,058) | (600,719) |  |  |  |
Accounting Standards Update 2016-09 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Accumulated deferred income taxes |  | 10,400 |  |  |  |
Retained Earnings |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Cumulative effect adjustment |  |  |  | 11,208 | $ 10,382 |
Retained Earnings | Accounting Standards Update 2016-09 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Cumulative effect adjustment |  | 10,400 |  |  |  |
Restricted Shares and Performance-Based Shares |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Unrecognized compensation expense | $ 3,500 |  |  |  |  |
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 | $ 4,200 | 4,900 | 4,400 |  |  |
Cumulative effect adjustment |  |  |  | 0 |  |
Accumulated deferred income taxes | (521,990) | (502,767) |  |  |  |
Public Service Company of New Mexico | Retained Earnings |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Cumulative effect adjustment |  |  |  | $ 11,208 |  |
Texas-New Mexico Power Company |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Compensation expense for stock-based arrangements | 2,200 | 2,200 | $ 1,800 |  |  |
Accumulated deferred income taxes | $ (140,151) | $ (136,238) |  |  |  |
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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