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SHARE-BASED PAYMENTS (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Share-based Payments [Line Items]      
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 $ 7,500 $ 7,000 $ 6,800
Share-based compensation, tax benefit 2,500 2,300 2,200
Fair value of shares granted (thousands of dollars) $ 3,660 3,005 3,001
Common Stock, Capital Shares Reserved for Future Issuance 4,300,000    
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 $ 3,600    
Total compensation cost not yet recognized, period for recognition 1 year 8 months 12 days    
Fair value of vested shares $ 3,400 $ 3,300 $ 3,300
Nonvested beginning balance (in units) 99,800    
Nonvested beginning balance (in dollars per unit) $ 82.29    
Granted (in units) 50,353    
Weighted -average grant date fair value (per unit) $ 72.69 $ 96.21 $ 83.94
Vested (in units) (42,821)    
Vested (in dollars per unit) $ 71.67    
Forfeited (in units) (13,058)    
Forfeited (in dollars per unit) $ 81.07    
Nonvested ending balance (in units) 94,274 99,800  
Nonvested ending balance (in dollars per unit) $ 82.16 $ 82.29  
Performance Unit Awards [Member]      
Share-based Payments [Line Items]      
Award vesting period 3 years    
Forfeiture rate maximum (in hundredths) 3.00%    
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.    
Total compensation cost not yet recognized $ 6,600    
Total compensation cost not yet recognized, period for recognition 1 year 8 months 12 days    
Fair value of vested shares $ 7,200 $ 10,200 $ 12,700
Nonvested beginning balance (in units) 208,520    
Nonvested beginning balance (in dollars per unit) $ 87.97    
Granted (in units) 107,381    
Weighted -average grant date fair value (per unit) $ 82.51 $ 102.77 $ 89.86
Vested (in units) (76,483)    
Vested (in dollars per unit) $ 74.04    
Forfeited (in units) (40,819)    
Forfeited (in dollars per unit) $ 89.19    
Nonvested ending balance (in units) 198,599 208,520  
Nonvested ending balance (in dollars per unit) $ 90.13 $ 87.97  
Fair value of shares granted (thousands of dollars) $ 8,860 $ 6,502 $ 6,401
Expected volatility rate 32.70% 16.40% 18.70%
Expected dividend yield 3.19% 2.25% 2.38%
Risk-free interest rate 0.20% 1.40% 2.50%
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.    
Common Stock, Capital Shares Reserved for Future Issuance 1,250,000    
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% 50.00% 44.00%
Shares sold under employee stock purchase plan 89,240 92,507 71,613
Share price of shares sold under Employee Stock Purchase Plan in dollars per share $ 63.41 $ 64.77 $ 71.42
Share-based Payment Arrangement, Expense $ 1,100 $ 1,100 $ 1,500