SHARE-BASED PAYMENTS (Details) - USD ($)
$ / shares in Units, $ in Thousands |
12 Months Ended |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
| 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 |
1,800,000
|
|
|
| Share-based compensation expense, net of tax |
$ 6,800
|
$ 6,100
|
$ 4,900
|
| Share-based compensation, tax benefit |
2,200
|
2,100
|
3,000
|
| Share-based compensation expense |
$ 9,314
|
8,195
|
$ 8,876
|
| 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.
|
|
|
| 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 |
$ 3,000
|
|
|
| Total compensation cost not yet recognized, period for recognition |
1 year 9 months 18 days
|
|
|
| Fair value of vested shares |
$ 3,300
|
$ 4,700
|
|
| Nonvested beginning balance (in units) |
109,506
|
|
|
| Nonvested beginning balance (in dollars per unit) |
$ 63.45
|
|
|
| Granted (in units) |
35,753
|
|
|
| Weighted -average grant date fair value (per unit) |
$ 83.94
|
$ 68.17
|
$ 63.97
|
| Vested (in units) |
(39,418)
|
|
|
| Vested (in dollars per unit) |
$ 58.60
|
|
|
| Forfeited (in units) |
(1,443)
|
|
|
| Forfeited (in dollars per unit) |
$ 69.87
|
|
|
| Nonvested ending balance (in units) |
104,398
|
109,506
|
|
| Nonvested ending balance (in dollars per unit) |
$ 72.21
|
$ 63.45
|
|
| Fair value of shares granted (thousands of dollars) |
$ 3,001
|
$ 2,583
|
$ 2,420
|
| Performance Unit Awards [Member] |
|
|
|
| Share-based Payments [Line Items] |
|
|
|
| Forfeiture rate maximum (in hundredths) |
3.00%
|
|
|
| Total compensation cost not yet recognized |
$ 6,400
|
|
|
| Total compensation cost not yet recognized, period for recognition |
1 year 9 months 18 days
|
|
|
| Fair value of vested shares |
$ 12,700
|
$ 13,700
|
|
| Nonvested beginning balance (in units) |
219,331
|
|
|
| Nonvested beginning balance (in dollars per unit) |
$ 69.21
|
|
|
| Granted (in units) |
71,237
|
|
|
| Weighted -average grant date fair value (per unit) |
$ 89.86
|
$ 74.04
|
$ 68.94
|
| Vested (in units) |
(70,548)
|
|
|
| Vested (in dollars per unit) |
$ 64.06
|
|
|
| Forfeited (in units) |
(1,844)
|
|
|
| Forfeited (in dollars per unit) |
$ 73.57
|
|
|
| Nonvested ending balance (in units) |
218,176
|
219,331
|
|
| Nonvested ending balance (in dollars per unit) |
$ 77.58
|
$ 69.21
|
|
| Fair value of shares granted (thousands of dollars) |
$ 6,401
|
$ 5,882
|
$ 5,110
|
| Expected volatility rate |
18.70%
|
18.80%
|
20.70%
|
| Expected dividend yield |
2.38%
|
2.70%
|
2.63%
|
| Risk-free interest rate |
2.50%
|
2.38%
|
1.48%
|
| 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 |
44.00%
|
45.00%
|
43.00%
|
| Shares sold under employee stock purchase plan |
71,613
|
76,231
|
78,472
|
| Share price of shares sold under Employee Stock Purchase Plan in dollars per share |
$ 71.42
|
$ 63.01
|
$ 56.80
|
| Employee Stock Purchase Plan Compensation Expense |
$ 1,500
|
$ 1,000
|
$ 1,200
|