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SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF PARENT 4 (Details)
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Note payable to banks      
Line of Credit Facility, Current Borrowing Capacity $ 200,000,000.0    
Maximum borrowing capacity 260,000,000.0    
Notes Payable to Bank 134,200,000    
Maximum borrowing capacity 260,000,000.0    
Short-term borrowing activities (excluding letters of credit)      
Notes payable to banks 0 $ 5,000,000  
Parent [Member]      
Note payable to banks      
Line of Credit Facility, Current Borrowing Capacity $ 30,000,000    
Short-term borrowing (excluding letters of credit)      
Short-term borrowing activities (excluding letters of credit)      
Interest Rate at the end of the period (as a percent)   2.44%  
Weighted Average Annual Interest Rate (as a percent) 1.47% 2.88%  
Maximum Amount Outstanding $ 249,200,000 $ 205,500,000  
Average Amount Outstanding $ 162,995,000 167,392,000  
Short-term borrowing (excluding letters of credit) | Minimum      
Short-term borrowing activities (excluding letters of credit)      
Interest Rate at the end of the period (as a percent) 1.19%    
Short-term borrowing (excluding letters of credit) | Maximum      
Short-term borrowing activities (excluding letters of credit)      
Interest Rate at the end of the period (as a percent) 1.90%    
Long-term Debt [Member]      
Short-term borrowing activities (excluding letters of credit)      
Notes Payable, Current $ 134,200,000 205,000,000  
Syndicated revolving credit facility      
Note payable to banks      
Variable rate basis Euro rate    
Syndicated revolving credit facility | Minimum      
Note payable to banks      
Interest coverage ratio 3.25    
Syndicated revolving credit facility | Maximum      
Note payable to banks      
Total funded debt ratio 0.65    
Parent Company      
Note payable to banks      
Line of Credit Facility, Current Borrowing Capacity $ 200,000,000.0    
Short-term borrowing activities (excluding letters of credit)      
Notes payable to banks $ 0 $ 5,000,000  
Parent Company | Short-term borrowing (excluding letters of credit)      
Short-term borrowing activities (excluding letters of credit)      
Interest Rate at the end of the period (as a percent) 1.19% 2.44%  
Weighted Average Annual Interest Rate (as a percent) 1.47% 2.88%  
Maximum Amount Outstanding $ 249,000,000 $ 205,500,000  
Average Amount Outstanding 160,495,000 167,392,000  
Parent Company | Long-term Debt [Member]      
Short-term borrowing activities (excluding letters of credit)      
Notes Payable, Current $ 114,000,000 205,000,000  
Parent Company | Syndicated revolving credit facility      
Note payable to banks      
Total funded debt ratio 0.47    
Interest coverage ratio 7.74    
Parent Company | Syndicated revolving credit facility | Minimum      
Note payable to banks      
Total funded debt ratio 0.65    
Interest coverage ratio 3.25    
Parent Company | Syndicated revolving credit facility | Maximum      
Note payable to banks      
Total funded debt ratio 1.00    
Parent Company | Syndicated revolving credit facility | Letters of credit      
Note payable to banks      
Maximum borrowing capacity $ 25,000,000.0    
Maximum borrowing capacity 25,000,000.0    
Letter of credit, amount $ 455,000    
Letter of credit fee (as a percent) 0.65%    
Subsidiaries [Member]      
Short-term borrowing activities (excluding letters of credit)      
Dividends paid $ (47,300,000) (42,700,000) $ (79,000,000.0)
GSWC      
Short-term borrowing activities (excluding letters of credit)      
Dividends paid $ (22,500,000) $ (20,200,000) $ (68,850,000)