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Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Regulatory Assets [Line Items]    
Net Regulatory Assets $ 100,810 $ 103,625
Total regulatory liability in Consolidated Balance Sheets 58,793 59,149
Deferred Income Tax Charge [Member]    
Regulatory Assets [Line Items]    
Regulatory Liabilities 58,793 59,149
Postretirement pensions and other medical benefits    
Regulatory Assets [Line Items]    
Regulatory assets: 66,233 66,233
Balancing and memorandum accounts, net    
Regulatory Assets [Line Items]    
Regulatory assets: 31,476 34,413
Other, net    
Regulatory Assets [Line Items]    
Regulatory assets: $ 3,101 $ 2,979