Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Net Regulatory Assets	$ 138,230	$ 120,417
Less: current regulatory asset, net	5,691	6,472
Total regulatory assets, net, less current portion	132,539	113,945
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory assets:	6,500	2,433
Postretirement pensions and other medical benefits
Regulatory Assets [Line Items]
Regulatory assets:	76,439	73,525
Business combinations debt premium, net [Member]
Regulatory Assets [Line Items]
Regulatory assets:	23,749	25,020
Balancing and memorandum accounts, net
Regulatory Assets [Line Items]
Regulatory assets:	17,615	5,283
WRA [Member]
Regulatory Assets [Line Items]
Regulatory assets:	7,466	9,108
Other, net
Regulatory Assets [Line Items]
Regulatory assets:	$ 6,461	$ 5,048