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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, 2017
Dec. 31, 2016
Accounting Policies [Abstract]    
Refund period for advances for construction received after 1981 40 years  
Estimated Refunds of Advances for Construction [Abstract]    
2016 $ 2,766  
2017 2,766  
2018 2,766  
2019 2,766  
2020 2,766  
Thereafter 52,338  
Customer Advances for Construction 83,695 $ 84,815
Customer Advances for Construction, Non-Refundable $ 17,527  
Amortization period for taxes paid relating to advances and contributions 40 years