Commitments (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) gal / mo ServiceLine	Jun. 30, 2017 USD ($)	Dec. 31, 2017 Round
Company-Owned Lead Service Lines [Member]
Commitments [Abstract]
Term to replace all remaining company-owned lead service lines			4 years
Costs incurred to replace company-owned lead service lines	$ 1,855		$ 1,855
Additional costs to be incurred to replace company-owned lead service lines	152		$ 152
Number of rounds of compliance testing completed at customer's tap | Round					2
Monthly credit provided to affected customers to flush line | gal / mo			200
Water testing expenses and flushing credits incurred	1	$ 4	$ 3	$ 11
Customer-Owned Lead Service Lines [Member]
Commitments [Abstract]
Number of lead customer-owned service lines to be replaced annually | ServiceLine			400
Term of tariff modification to replace customer-owned lead service lines regardless of material used for company-owned service lines			9 years
Term of tariff modification to replace customer-owned lead service lines connected to company-owned lead service lines			4 years
Costs incurred to replace customer-owned lead service lines under four-year tariff modification	235		$ 235
Additional costs to be incurred to replace customer-owned lead service lines under four-year tariff modification	18		18
Costs to be incurred to replace customer-owned lead service lines under nine-year tariff modification	$ 1,040		$ 1,040
Customer-Owned Lead Service Lines [Member] | Minimum [Member]
Commitments [Abstract]
Recovery period of regulatory asset			4 years
Customer-Owned Lead Service Lines [Member] | Maximum [Member]
Commitments [Abstract]
Recovery period of regulatory asset			6 years