Derivative and Hedging Instruments (Details-4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Current and Long-Term Regulatory Asset [Member]
Change in Derivative Fair Value [Roll Forward]
Beginning balance	$ 49,281	$ 54,082
Unrealized (gain) loss	(2,743)	1,575
Realized (loss) gain reclassified to a deferred account	(1,260)	(1,060)
Realized loss reclassified to income statement	(3,320)	(5,316)
Ending balance	41,958	49,281
Other Current Assets [Member]
Change in Derivative Fair Value [Roll Forward]
Beginning balance	230	1,208
Unrealized (gain) loss	0	0
Realized (loss) gain reclassified to a deferred account	1,260	1,060
Realized loss reclassified to income statement	(684)	(2,038)
Ending balance	$ 806	$ 230