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Stock-Based Compensation - Restricted Stock Awards (Details) - USD ($) |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 02, 2018 | Mar. 03, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Feb. 28, 2017 | Dec. 31, 2015 | Jan. 01, 2015 | Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  | $ 7,100,000 | $ 6,200,000 | $ 5,600,000 |  |  |  |
Chairman, President, and Chief Executive Officer | Common Stock | Achieves a specified improvement in total shareholder return at the end of 2016 compared to 2011 and she remains an employee |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  |  |  |  |  |  | 135,000
Chairman, President, and Chief Executive Officer | Common Stock | Achieves Specified Improvement In Total Shareholder Return At End Of 2014 Compared To 2011 And She Remains Employee |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  |  |  |  |  |  | 35,000
Number of shares approved upon meeting target (in shares) |  |  |  |  |  | 100,000 |  |  |
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) |  |  |  |  |  |  | 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 |  |
Executive Vice President and Chief Financial Officer | Common Stock | Achieved performance target for 2015 and 2016 |  |  |  |  |  |  |  |  |
Restricted Stock |  |  |  |  |  |  |  |  |
Granted (in shares) | 7,670 | 2,754 |  |  |  |  |  |  |
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 |
Restricted Stock |  |  |  |  |  |  |  |  |
Restricted Stock |  |  |  |  |  |  |  |  |
Beginning balance (in shares) |  |  | 189,045 |  |  |  |  |  |
Granted (in shares) |  |  | 221,062 |  |  |  |  |  |
Exercised (in shares) |  |  | (237,402) |  |  |  |  |  |
Forfeited (in shares) |  |  | (6,054) |  |  |  |  |  |
Ending balance (in shares) |  |  | 166,651 | 189,045 |  |  |  |  |
Weighted-Average Grant Date Fair Value |  |  |  |  |  |  |  |  |
Beginning balance (in dollars per share) |  |  | $ 31.11 |  |  |  |  |  |
Granted (in dollars per share) |  |  | 29.65 | $ 23.06 | $ 26.49 |  |  |  |
Exercised (in dollars per share) |  |  | 28.46 |  |  |  |  |  |
Forfeited (in dollars per share) |  |  | 31.37 |  |  |  |  |  |
Ending balance (in dollars per share) |  |  | 32.93 | 31.11 |  |  |  |  |
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) |  |  | $ 29.65 | $ 23.06 | $ 26.49 |  |  |  |
Total fair value of restricted shares that vested |  |  | $ 8,558,000 | $ 5,747,000 | $ 5,079,000 |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |
Beginning balance (in shares) |  |  | 193,441 |  |  |  |  |  |
Granted (in shares) |  |  | 0 |  |  |  |  |  |
Exercised (in shares) |  |  | (112,441) |  |  |  |  |  |
Forfeited (in shares) |  |  | 0 |  |  |  |  |  |
Expired (in shares) |  |  | 0 |  |  |  |  |  |
Ending balance (in shares) |  |  | 81,000 | 193,441 |  |  |  |  |
Weighted Average Exercise Price |  |  |  |  |  |  |  |  |
Beginning balance (in dollars per share) |  |  | $ 9.98 |  |  |  |  |  |
Granted (in dollars per share) |  |  | 0.00 |  |  |  |  |  |
Exercised (in dollars per share) |  |  | 8.56 |  |  |  |  |  |
Forfeited (in dollars per share) |  |  | 0.00 |  |  |  |  |  |
Expired (in dollars per share) |  |  | 0.00 |  |  |  |  |  |
Ending balance (in dollars per share) |  |  | $ 11.94 | $ 9.98 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Aggregate intrinsic value of stock options outstanding |  |  | $ 2,400,000 |  |  |  |  |  |
Weighted average remaining contract life |  |  | 1 year 15 days |  |  |  |  |  |
Exercise price of stock options that are greater than the closing price (in shares) |  |  | 0 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Weighted-average grant date fair value options granted (in dollars per share) |  |  | $ 0.00 | $ 0.00 | $ 0.00 |  |  |  |
Total fair value of options that vested |  |  | $ 0 | $ 0 | $ 0 |  |  |  |
Total intrinsic value of options exercised |  |  | 3,117,000 | 2,234,000 | 1,242,000 |  |  |  |
Performance Shares |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  | $ 1,000,000 |  |  |  |  |  |
Performance Shares | Executive |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Shares granted (in shares) |  |  | 97,697 |  |  |  |  |  |
Weighted percentage assigned to achieving market targets |  |  | 40.00% |  |  |  |  |  |
Weighted percentage assigned to achieving performance targets |  |  | 60.00% |  |  |  |  |  |
Shares excluded (in shares) |  |  | 47,279 |  |  |  |  |  |
Shares approved, maximum number of shares available in year two (in shares) |  |  | 130,302 |  |  |  |  |  |
Shares approved, maximum number of shares available in year three (in shares) |  |  | 146,941 |  |  |  |  |  |
Performance period |  |  | 3 years |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |
Additional pre tax expense |  |  | $ 4,900,000 | 4,400,000 | 4,200,000 |  |  |  |
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 |  |  | 2,200,000 | $ 1,800,000 | $ 1,500,000 |  |  |  |
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 Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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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=109197908&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=109197908&loc=d3e5070-113901 + Details
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- Definition The number of grants made 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=109197908&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=109197908&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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=109197908&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + 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=109197908&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested 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)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=109197908&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=109197908&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=109197908&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + 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=109197908&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + 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=109197908&loc=d3e5070-113901 + Details
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- Definition Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements. + 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)(4) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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