Derivative and Hedging Instruments (Details-4) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Current and Long-Term Regulatory Asset [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	$ 45,316	$ 50,521	$ 49,281	$ 54,082
Unrealized (gain) loss	(1,277)	(663)	(3,698)	1,128
Realized (loss) gain reclassified to a deferred account	(313)	17	(935)	(1,417)
Realized (loss) gain reclassified to income statement	(1,450)	(304)	(2,372)	(4,222)
Ending balance,	42,276	49,571	42,276	49,571
Other Current Assets [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	618	659	230	1,208
Unrealized (gain) loss	0	0	0	0
Realized (loss) gain reclassified to a deferred account	313	(17)	935	1,417
Realized (loss) gain reclassified to income statement	(57)	46	(291)	(1,937)
Ending balance,	$ 874	$ 688	$ 874	$ 688