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SHARE-BASED PAYMENTS (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
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 1,900,000    
Share-based compensation expense, net of tax $ 7,000 $ 6,800 $ 6,100
Share-based compensation, tax benefit 2,300 2,200 2,100
Share-based compensation expense $ 9,803 9,314 8,195
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 $ 3,100    
Total compensation cost not yet recognized, period for recognition 1 year 8 months 12 days    
Fair value of vested shares $ 3,300 $ 3,300 $ 4,700
Nonvested beginning balance (in units) 104,398    
Nonvested beginning balance (in dollars per unit) $ 72.21    
Granted (in units) 31,233    
Weighted -average grant date fair value (per unit) $ 96.21 $ 83.94 $ 68.17
Vested (in units) (34,066)    
Vested (in dollars per unit) $ 64.32    
Forfeited (in units) (1,765)    
Forfeited (in dollars per unit) $ 78.89    
Nonvested ending balance (in units) 99,800 104,398  
Nonvested ending balance (in dollars per unit) $ 82.29 $ 72.21  
Fair value of shares granted (thousands of dollars) $ 3,005 $ 3,001 $ 2,583
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 $ 6,700    
Total compensation cost not yet recognized, period for recognition 1 year 8 months 12 days    
Fair value of vested shares $ 10,200 $ 12,700 $ 13,700
Nonvested beginning balance (in units) 218,176    
Nonvested beginning balance (in dollars per unit) $ 77.58    
Granted (in units) 63,268    
Weighted -average grant date fair value (per unit) $ 102.77 $ 89.86 $ 74.04
Vested (in units) (69,107)    
Vested (in dollars per unit) $ 68.94    
Forfeited (in units) (3,817)    
Forfeited (in dollars per unit) $ 84.10    
Nonvested ending balance (in units) 208,520 218,176  
Nonvested ending balance (in dollars per unit) $ 87.97 $ 77.58  
Fair value of shares granted (thousands of dollars) $ 6,502 $ 6,401 $ 5,882
Expected volatility rate 16.40% 18.70% 18.80%
Expected dividend yield 2.25% 2.38% 2.70%
Risk-free interest rate 1.40% 2.50% 2.38%
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 50.00% 44.00% 45.00%
Shares sold under employee stock purchase plan 92,507 71,613 76,231
Share price of shares sold under Employee Stock Purchase Plan in dollars per share $ 64.77 $ 71.42 $ 63.01
Employee Stock Purchase Plan Compensation Expense $ 1,100 $ 1,500 $ 1,000