Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Net Regulatory Assets	$ 104,670	$ 99,554
Total regulatory liability in Consolidated Balance Sheets	60,650	62,476
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	60,650	62,476
Postretirement pensions and other medical benefits
Regulatory Assets [Line Items]
Regulatory assets:	68,556	68,556
Balancing and memorandum accounts, net
Regulatory Assets [Line Items]
Regulatory assets:	32,748	27,925
Other, net
Regulatory Assets [Line Items]
Regulatory assets:	$ 3,366	$ 3,073