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SHARE-BASED PAYMENTS (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Share-based Payments [Line Items] |  |  |
Common Stock, Capital Shares Reserved for Future Issuance | 4,300,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized | 1,800,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | 2,100,000 |  |
Share-based compensation expense, net of tax | $ 6,100 | $ 4,900 | $ 7,000
Share-based compensation, tax benefit | 2,100 | 3,000 | 4,300
Share-based compensation expense | $ 8,195 | $ 8,876 | $ 11,219
Employee Stock Purchase Plan [Member] |  |  |
Share-based Payments [Line Items] |  |  |
Description | Subject to certain exclusions, all employees who work at least 20 hours per week are eligible to participate in the ESPP. Employees can choose to have up to 10 percent of their annual base pay withheld to purchase our common stock, subject to terms and limitations of the plan. The purchase price of the stock is 85 percent of the lower of the average market price of our common stock on the grant date or exercise date. |  |
Employee Stock Award Program [Member] |  |  |
Share-based Payments [Line Items] |  |  |
Description |  | Under the program, each time the per-share closing price of our common stock on the NYSE closed for the first time at or above each $1.00 increment above its previous historical high closing price, we issued, for no monetary consideration, one share of our common stock to all eligible employees. |
Restricted Stock Units (RSUs) [Member] |  |  |
Share-based Payments [Line Items] |  |  |
Award vesting period | 3 years |  |
Forfeiture rate maximum (in hundredths) | 3.00% |  |
Total compensation cost not yet recognized | $ 2,600 |  |
Total compensation cost not yet recognized, period for recognition | 1 year 8 months 15 days |  |
Fair value of vested shares | $ 4,700 | $ 5,500 |
Nonvested beginning balance (in units) | 140,665 |  |
Nonvested beginning balance (in dollars per unit) | $ 51.97 |  |
Granted (in units) | 37,893 |  |
Weighted -average grant date fair value (per unit) | $ 68.17 | $ 63.97 | $ 58.30
Vested (in units) | (66,543) |  |
Vested (in dollars per unit) | $ 41.92 |  |
Forfeited (in units) | (2,509) |  |
Forfeited (in dollars per unit) | $ 62.44 |  |
Nonvested ending balance (in units) | 109,506 | 140,665 |
Nonvested ending balance (in dollars per unit) | $ 63.45 | $ 51.97 |
Fair value of shares granted (thousands of dollars) | $ 2,583 | $ 2,420 | $ 2,503
Performance Unit Awards [Member] |  |  |
Share-based Payments [Line Items] |  |  |
Award vesting period | 3 years |  |
Forfeiture rate maximum (in hundredths) | 3.00% |  |
Total compensation cost not yet recognized | $ 5,800 |  |
Total compensation cost not yet recognized, period for recognition | 1 year 9 months 15 days |  |
Fair value of vested shares | $ 13,700 | $ 15,600 |
Nonvested beginning balance (in units) | 237,324 |  |
Nonvested beginning balance (in dollars per unit) | $ 57.78 |  |
Granted (in units) | 79,447 |  |
Weighted -average grant date fair value (per unit) | $ 74.04 | $ 68.94 | $ 64.06
Vested (in units) | (93,976) |  |
Vested (in dollars per unit) | $ 44.48 |  |
Forfeited (in units) | (3,464) |  |
Forfeited (in dollars per unit) | $ 67.97 |  |
Nonvested ending balance (in units) | 219,331 | 237,324 |
Nonvested ending balance (in dollars per unit) | $ 69.21 | $ 57.78 |
Fair value of shares granted (thousands of dollars) | $ 5,882 | $ 5,110 | $ 4,766
Expected volatility rate | 18.80% | 20.70% | 18.20%
Expected dividend yield | 2.70% | 2.63% | 2.40%
Risk-free interest rate | 2.38% | 1.48% | 0.91%
Description | Upon vesting, a holder of performance stock units is entitled to receive a number of shares of common stock equal to a percentage (0 percent to 200 percent) of the performance stock units granted, based on our total shareholder return over the vesting period, compared with the total shareholder return of a peer group of other utilities over the same period. |  |
Employee Stock Purchase Plan [Member] |  |  |
Share-based Payments [Line Items] |  |  |
Common Stock, Capital Shares Reserved for Future Issuance | 700,000 |  |
Maximum allowable percentage of annual base pay withheld to purchase common stock | 10.00% |  |
Purchase price percentage of the lower of its grant date or exercise date market price (in hundredths) | 85.00% |  |
Percent of employees who participated in the Employee Stock Purchase Plan | 45.00% | 43.00% | 41.00%
Shares sold under employee stock purchase plan | 76,231 | 78,472 | 83,431
Share price of shares sold under Employee Stock Purchase Plan in dollars per share | $ 63.01 | $ 56.80 | $ 54.51
Employee Stock Purchase Plan Compensation Expense | $ 1,000 | $ 1,200 | $ 1,400
Employee Stock Award Program [Member] |  |  |
Share-based Payments [Line Items] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized |  | 125,000 |
Shares issued to employees under the Employee Stock Award Program |  | 13,791 | 50,573
Share-based compensation expense |  | $ 900 | $ 3,000

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- Definition Employees can choose to have up to 10 percent of their annual base pay withheld to purchase our common stock, subject to terms and limitations of the plan. + References No definition available. + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition General descriptive information regarding an arrangement under which employees and directors receive: (a) awards of equity shares, equity share options, or other equity instruments; or (b) the entity incurs liabilities to them: (1) in amounts based, at least in part, on the price of the entity's shares or other equity instruments; or (2) that require or may require settlement by issuance of the entity's shares. Such an arrangement is usually provided to employees and directors to compensate them, provide performance incentives to them, and to attract or retain their services. May also include pertinent information particular to a plan that is not elsewhere specified in the taxonomy. + 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 (a) -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 50 -Paragraph 2 -Subparagraph (a) -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 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 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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=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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