Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Regulatory Assets [Line Items]
Net income (loss) before noncontrolling interest	$ 12,871	$ 20,584	$ 14,156	$ 24,255
Net Regulatory Assets	98,332		98,332		$ 99,554
Total regulatory liability in Consolidated Balance Sheets	61,639		61,639		62,476
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	61,639		61,639		62,476
Postretirement pensions and other medical benefits
Regulatory Assets [Line Items]
Regulatory assets:	68,556		68,556		68,556
Balancing and memorandum accounts, net
Regulatory Assets [Line Items]
Regulatory assets:	26,526		26,526		27,925
Other, net
Regulatory Assets [Line Items]
Regulatory assets:	$ 3,250		$ 3,250		$ 3,073