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Stock-Based Compensation - Performance Equity Plan and Accounting for Stock Awards (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | 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 | $ 7,100 | $ 6,200 | $ 5,600 |  |  |
Tax expense (benefit) related to stock compensation awards | 4,647 | (2,324) | 0 |  |  |
Cumulative effect adjustment (Note 9) |  |  |  | $ 0 | $ 10,382 |
Accounting Standards Update 2016-09 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Tax expense (benefit) related to stock compensation awards | 1,400 | 2,300 |  |  |  |
Retained Earnings |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Cumulative effect adjustment (Note 9) |  | 11,200 |  | 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 (Note 9) |  | 10,400 |  |  |  |
Restricted Shares and Performance-Based Shares |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Unrecognized compensation expense | $ 4,000 |  |  |  |  |
Period to recognize compensation expense | 1 year 5 months 12 days |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Compensation expense for stock-based arrangements | $ 4,900 | 4,400 | 4,200 |  |  |
Tax expense (benefit) related to stock compensation awards | 3,967 | (1,708) | 0 |  |  |
Public Service Company of New Mexico | Accounting Standards Update 2016-09 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Tax expense (benefit) related to stock compensation awards | 1,000 | 1,700 |  |  |  |
Public Service Company of New Mexico | Retained Earnings |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Cumulative effect adjustment (Note 9) |  |  |  | $ 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 | 1,800 | 1,500 |  |  |
Tax expense (benefit) related to stock compensation awards | 735 | (616) | $ 0 |  |  |
Texas-New Mexico Power Company | Accounting Standards Update 2016-09 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Tax expense (benefit) related to stock compensation awards | $ 400 | $ 600 |  |  |  |
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) |  |  |  |  |  | 13,500,000.0
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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