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Stock-Based Compensation - Restricted Stock Awards (Details) - USD ($) |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 02, 2018 | Mar. 03, 2017 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Feb. 28, 2018 | Feb. 28, 2017 | Dec. 31, 2015 | Jan. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  | $ 8,100,000 | $ 6,400,000 | $ 7,100,000 |  |  |  |
Executive Vice President and Chief Financial Officer | Common Stock | Achieved performance target for 2015 and 2016 |  |  |  |  |  |  |  |  |
Restricted Stock, Shares |  |  |  |  |  |  |  |  |
Granted (in shares) | 7,670 | 2,754 |  |  |  |  |  |  |
Restricted Stock, Weighted-Average Grant Date Fair Value |  |  |  |  |  |  |  |  |
Granted (in dollars per share) | $ 35.85 | $ 36.30 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Number of shares approved upon meeting target (in shares) |  |  |  |  |  |  | 100,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Weighted-average grant date fair value (in dollars per share) | $ 35.85 | $ 36.30 |  |  |  |  |  |  |
Executive Vice President and Chief Financial Officer | Common Stock | Achieved performance target for 2015, 2016 and 2017 |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Value of stock to be granted if performance targets are achieved |  |  |  |  |  |  |  |  | $ 275,000
Chairman, President, and Chief Executive Officer | Common Stock | Achieves a specific performance target by the end of 2019 and she remains an employee |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  |  |  | 35,906 |  | 53,859 |
Chairman, President, and Chief Executive Officer | Common Stock | Achieves a specific performance target by the end of 2017 and she remains an employee |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  |  |  | 17,953 |  |  |
Restricted Stock |  |  |  |  |  |  |  |  |
Restricted Stock, Shares |  |  |  |  |  |  |  |  |
Beginning balance (in shares) |  |  | 161,542 |  |  |  |  |  |
Granted (in shares) |  |  | 246,029 |  |  |  |  |  |
Exercised (in shares) |  |  | (238,054) |  |  |  |  |  |
Forfeited (in shares) |  |  | (1,456) |  |  |  |  |  |
Ending balance (in shares) |  |  | 168,061 | 161,542 |  |  |  |  |
Restricted Stock, Weighted-Average Grant Date Fair Value |  |  |  |  |  |  |  |  |
Beginning balance (in dollars per share) |  |  | $ 38.21 |  |  |  |  |  |
Granted (in dollars per share) |  |  | 36.73 | $ 37.92 | $ 29.65 |  |  |  |
Exercised (in dollars per share) |  |  | 34.86 |  |  |  |  |  |
Forfeited (in dollars per share) |  |  | 41.32 |  |  |  |  |  |
Ending balance (in dollars per share) |  |  | 40.77 | 38.21 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Weighted-average grant date fair value (in dollars per share) |  |  | $ 36.73 | $ 37.92 | $ 29.65 |  |  |  |
Total fair value of restricted shares that vested |  |  | $ 8,299,000 | $ 6,246,000 | $ 8,558,000 |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |
Stock Options, Shares |  |  |  |  |  |  |  |  |
Beginning balance (in shares) |  |  | 2,000 |  |  |  |  |  |
Granted (in shares) |  |  | 0 |  |  |  |  |  |
Exercised (in shares) |  |  | (2,000) |  |  |  |  |  |
Forfeited (in shares) |  |  | 0 |  |  |  |  |  |
Ending balance (in shares) |  |  | 0 | 2,000 |  |  |  |  |
Options, Weighted Average Exercise Price |  |  |  |  |  |  |  |  |
Beginning balance (in dollars per share) |  |  | $ 12.22 |  |  |  |  |  |
Granted (in dollars per share) |  |  | 0 |  |  |  |  |  |
Exercised (in dollars per share) |  |  | 12.22 |  |  |  |  |  |
Forfeited (in dollars per share) |  |  | 0 |  |  |  |  |  |
Ending balance (in dollars per share) |  |  | $ 0 | $ 12.22 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Aggregate intrinsic value of stock options outstanding |  |  |  | $ 100,000 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Total intrinsic value of options exercised |  |  | $ 84,000 | 2,617,000 | 3,117,000 |  |  |  |
Performance Shares |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  |  |  | 1,000,000.0 |  |  |  |
Performance Shares | Executive |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Shares excluded in year two (in shares) |  |  | 150,543 |  |  |  |  |  |
Shares excluded in year three (in shares) |  |  | 142,080 |  |  |  |  |  |
Performance period |  |  | 3 years |  |  |  |  |  |
Performance Shares | Executive | Achieved performance target for 2016 through 2018 |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Shares excluded (in shares) |  |  | 47,279 |  |  |  |  |  |
Weighted percentage assigned to achieving market targets |  |  | 40.00% |  |  |  |  |  |
Weighted percentage assigned to achieving performance targets |  |  | 60.00% |  |  |  |  |  |
Performance Shares | Executive | Achieved performance target for 2017 through 2019 |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Shares excluded (in shares) |  |  | 122,277 |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  | $ 5,500,000 | 4,200,000 | 4,900,000 |  |  |  |
Public Service Company of New Mexico | Performance Shares |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  |  |  | 700,000 |  |  |  |
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  | $ 2,600,000 | $ 2,200,000 | 2,200,000 |  |  |  |
Texas-New Mexico Power Company | Performance Shares |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  |  |  | $ 300,000 |  |  |  |

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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(3) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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