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Summary of Significant Accounting Policies - Advances for Construction and Contributions in Aid of Construction (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Accounting Policies [Abstract]    
Refund period for advances for construction received after 1981 40 years  
Estimated Refunds of Advances for Construction [Abstract]    
2021 $ 3,050  
2022 3,050  
2023 3,038  
2024 2,958  
2025 2,896  
Thereafter 52,462  
Advances for construction 125,027 $ 112,339
Customer advances for construction, non-refundable 29,117  
Customer advances for construction, refunded on service connections $ 28,456  
Contract with customer, refund period 40 years  
Contract with customer, fair value of advances $ 50,391 $ 50,035
Amortization period for taxes paid relating to advances and contributions 40 years