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Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Regulatory Assets [Line Items]    
Net Regulatory Assets $ 81,273 $ 103,625
Less: current regulatory asset, net 7,493 26,910
Total regulatory assets, net, less current portion 73,780 76,715
Total regulatory liability in Consolidated Balance Sheets 56,936 59,149
Deferred Income Tax Charge [Member]    
Regulatory Assets [Line Items]    
Regulatory Liabilities 56,936 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: 11,608 34,413
Other, net    
Regulatory Assets [Line Items]    
Regulatory assets: $ 3,432 $ 2,979