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Commitments and Contingent Liabilities (Narrative) (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
gal
Dec. 31, 2017
USD ($)
Purchase Commitment, Excluding Long-term Commitment [Line Items]    
Guaranty liabilty for AWM's performance | $ $ 100 $ 100
Budgeted construction cost for construction program, 2018 | $ $ 78,000  
Period of wastewater agreement 20 years  
Rental expenses under operating leases | $ $ 100 $ 100
NJ Water Supply Authority [Member]    
Purchase Commitment, Excluding Long-term Commitment [Line Items]    
Purchase commitment expiration date of contract Nov. 30, 2023  
Water purchase per commitment | gal 27,000,000  
Regulated Water Authority [Member]    
Purchase Commitment, Excluding Long-term Commitment [Line Items]    
Purchase commitment expiration date of contract Feb. 27, 2021  
Water purchase per commitment | gal 3,000,000  
City of Dover [Member]    
Purchase Commitment, Excluding Long-term Commitment [Line Items]    
Water purchase per commitment | gal 15,000,000