Derivative and Hedging Instruments (Details-4) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Current and Long-Term Regulatory Asset [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	$ 48,073	$ 56,884	$ 49,281	$ 54,082
Unrealized (gain) loss	(1,731)	(4,580)	(2,420)	1,791
Realized (loss) gain reclassified to a deferred account	(544)	17	(622)	(1,434)
Realized (loss) gain reclassified to income statement	(482)	(1,800)	(923)	(3,918)
Ending balance,	45,316	50,521	45,316	50,521
Other Current Assets [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	142	655	230	1,208
Unrealized (gain) loss	0	0	0	0
Realized (loss) gain reclassified to a deferred account	544	(17)	622	1,434
Realized (loss) gain reclassified to income statement	(68)	21	(234)	(1,983)
Ending balance,	$ 618	$ 659	$ 618	$ 659