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Stock-Based Compensation (Details) - USD ($) |  |  | 3 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 02, 2018 | Mar. 03, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Feb. 28, 2018 | Dec. 31, 2017 | Feb. 28, 2017 | Jan. 01, 2017 | Mar. 31, 2015 | Jan. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Number of shares granted (in shares) |  |  | 0 |  |  |  |  |  |  |
Vesting period |  |  | 1 year |  |  |  |  |  |  |
Unrecognized expense related to stock awards |  |  | $ 5,800,000 |  |  | $ 3,800,000 |  |  |  |
Cumulative effect adjustment |  |  |  |  |  | (11,208,000) |  |  |  |
Restricted Stock, Shares |  |  |  |  |  |  |  |  |  |
Expired (in shares) |  |  | 0 |  |  |  |  |  |  |
Restricted Stock, Weighted- Average Grant Date Fair Value |  |  |  |  |  |  |  |  |  |
Expired (in dollars per share) |  |  | $ 0.00 |  |  |  |  |  |  |
Stock Options, Shares |  |  |  |  |  |  |  |  |  |
Outstanding at beginning of period (in shares) |  |  | 193,441 |  |  |  |  |  |  |
Granted (in shares) |  |  | 0 |  |  |  |  |  |  |
Exercised (in shares) |  |  | (97,941) |  |  |  |  |  |  |
Forfeited (in shares) |  |  | 0 |  |  |  |  |  |  |
Expired (in shares) |  |  | 0 |  |  |  |  |  |  |
Outstanding at end of period (in shares) |  |  | 95,500 |  |  |  |  |  |  |
Stock Options, Weighted- Average Exercise Price |  |  |  |  |  |  |  |  |  |
Outstanding at beginning of period (in dollars per share) |  |  | $ 9.98 |  |  |  |  |  |  |
Granted (in dollars per share) |  |  | 0.00 |  |  |  |  |  |  |
Exercised (in dollars per share) |  |  | 8.19 |  |  |  |  |  |  |
Forfeited (in dollars per share) |  |  | 0.00 |  |  |  |  |  |  |
Expired (in dollars per share) |  |  | 0.00 |  |  |  |  |  |  |
Outstanding at end of period (in dollars per share) |  |  | 11.81 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |  |
Weighted-average grant date fair value of options granted (in dollars per share) |  |  | $ 0.00 | $ 0.00 |  |  |  |  |  |
Total fair value of options that vested |  |  | $ 0 | $ 0 |  |  |  |  |  |
Total intrinsic value of options exercised |  |  | $ 2,711,000 | $ 945,000 |  |  |  |  |  |
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 [Line Items] |  |  |  |  |  |  |  |  |  |
Shares received if achieves specified improvement in total shareholders return (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 [Line Items] |  |  |  |  |  |  |  |  |  |
Shares received if achieves specified improvement in total shareholders return (in shares) |  |  |  |  |  |  |  |  | 17,953 |
Executive Vice President and Chief Financial Officer | Common Stock | Achieved performance target for 2015 and 2016 |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Value of stock awarded |  |  |  |  |  |  |  |  |  | $ 100,000
Number of shares available for grant |  |  |  |  | 275,000 |  | 100,000 |  |  |
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] |  |  |  |  |  |  |  |  |  |
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 [Line Items] |  |  |  |  |  |  |  |  |  |
Value of stock awarded |  |  |  |  |  |  |  |  |  | $ 275,000
Retained Earnings |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative effect adjustment |  |  |  |  |  | 11,208,000 |  |  |  |
Accounting Standards Update 2016-09 |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Accumulated deferred income taxes |  |  |  |  |  |  |  | $ (10,400,000) |  |
PNM |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative effect adjustment |  |  |  |  |  | (11,208,000) |  |  |  |
PNM | Retained Earnings |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative effect adjustment |  |  |  |  |  | $ 11,208,000 |  |  |  |
Restricted Shares and Performance Based Shares |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Period of time stock expense is expected to be recognized |  |  | 2 years 14 days | 1 year 6 months 11 days |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |  |  |  |  |  |  |  |
Expected quarterly dividends per share (in dollars per share) |  |  | $ 0.2650 | $ 0.2425 |  |  |  |  |  |
Risk-free interest rate |  |  | 2.39% | 1.58% |  |  |  |  |  |
Restricted Stock |  |  |  |  |  |  |  |  |  |
Restricted Stock, Shares |  |  |  |  |  |  |  |  |  |
Outstanding at beginning of period (in shares) |  |  | 189,045 |  |  |  |  |  |  |
Granted (in shares) |  |  | 204,654 |  |  |  |  |  |  |
Exercised (in shares) |  |  | (201,162) |  |  |  |  |  |  |
Forfeited (in shares) |  |  | (3,562) |  |  |  |  |  |  |
Expired (in shares) |  |  | 0 |  |  |  |  |  |  |
Outstanding at end of period (in shares) |  |  | 188,975 |  |  |  |  |  |  |
Restricted Stock, Weighted- Average Grant Date Fair Value |  |  |  |  |  |  |  |  |  |
Outstanding at beginning of period (in dollars per share) |  |  | $ 31.11 |  |  |  |  |  |  |
Granted (in dollars per share) |  |  | 29.03 | $ 22.12 |  |  |  |  |  |
Exercised (in dollars per share) |  |  | 27.68 |  |  |  |  |  |  |
Forfeited (in dollars per share) |  |  | 30.58 |  |  |  |  |  |  |
Expired (in dollars per share) |  |  | 0.00 |  |  |  |  |  |  |
Outstanding at end of period (in dollars per share) |  |  | $ 32.52 |  |  |  |  |  |  |
Stock Options, Shares |  |  |  |  |  |  |  |  |  |
Expired (in shares) |  |  | 0 |  |  |  |  |  |  |
Stock Options, Weighted- Average Exercise Price |  |  |  |  |  |  |  |  |  |
Expired (in dollars per share) |  |  | $ 0.00 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |  |  |  |  |  |  |
Weighted-average grant date fair value (in dollars per share) |  |  | $ 29.03 | $ 22.12 |  |  |  |  |  |
Total fair value of restricted shares that vested |  |  | $ 7,162,000 | $ 5,024,000 |  |  |  |  |  |
Performance Shares | Executive |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Shares granted (in shares) |  |  | 97,697 |  |  |  |  |  |  |
Shares exercised (in shares) |  |  | 49,682 |  |  |  |  |  |  |
Weighted percentage assigned to achieving market targets |  |  | 60.00% |  |  |  |  |  |  |
Weighted percentage assigned to achieving performance targets |  |  | 40.00% |  |  |  |  |  |  |
Maximum number of shares awarded in year 1 (in shares) |  |  | 134,219 |  |  |  |  |  |  |
Maximum number of shares awarded in year 2 (in shares) |  |  | 155,291 |  |  |  |  |  |  |
Maximum number of shares awarded in year 3 (in shares) |  |  | 152,750 |  |  |  |  |  |  |
Performance period |  |  | 3 years |  |  |  |  |  |  |
Market-Based Shares |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 2.36% | 1.54% |  |  |  |  |  |
Dividend yield |  |  | 2.96% | 2.67% |  |  |  |  |  |
Expected volatility |  |  | 19.12% | 20.80% |  |  |  |  |  |
Stock options |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate intrinsic value of stock options outstanding |  |  | $ 2,500,000 |  |  |  |  |  |  |
Weighted-average remaining contract life |  |  | 1 year 9 months 18 days |  |  |  |  |  |  |
Number of outstanding stock options with an exercise price greater than the closing price (in shares) |  |  | 0 |  |  |  |  |  |  |
Performance Equity Plan |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Vesting period |  |  | 3 years |  |  |  |  |  |  |
Vesting rate |  |  | 100.00% |  |  |  |  |  |  |

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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://www.xbrl.org/2003/role/presentationRef -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=96867065&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://www.xbrl.org/2003/role/presentationRef -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=96867065&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://www.xbrl.org/2003/role/presentationRef -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=96867065&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -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=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -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=96867065&loc=d3e5070-113901 + Details
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- Definition Net number of share options (or share units) granted during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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