Derivative and Hedging Instruments (Details-4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Current and Long-Term Regulatory Asset [Member]
Change in Derivative Fair Value [Roll Forward]
Beginning balance	$ 41,958	$ 49,281
Unrealized (gain) loss	(11,094)	(2,743)
Realized (loss) gain reclassified to a deferred account	523	(1,260)
Realized loss reclassified to income statement	443	(3,320)
Ending balance	31,830	41,958
Other Current Assets [Member]
Change in Derivative Fair Value [Roll Forward]
Beginning balance	806	230
Unrealized (gain) loss	0	0
Realized (loss) gain reclassified to a deferred account	(523)	1,260
Realized loss reclassified to income statement	94	(684)
Ending balance	$ 377	$ 806