Derivative and Hedging Instruments (Details-4) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Current and Long-Term Regulatory Asset [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	$ 41,241	$ 48,073	$ 41,958	$ 49,281
Unrealized loss (gain)	(4,027)	(1,731)	(3,899)	(2,420)
Realized (loss) gain reclassified to a deferred account	(363)	(544)	(521)	(622)
Realized gain (loss) reclassified to income statement	73	(482)	(614)	(923)
Ending balance,	36,924	45,316	36,924	45,316
Other Current Assets [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	319	142	806	230
Unrealized loss (gain)	0	0	0	0
Realized (loss) gain reclassified to a deferred account	363	544	521	622
Realized gain (loss) reclassified to income statement	(42)	(68)	(687)	(234)
Ending balance,	$ 640	$ 618	$ 640	$ 618