Derivative and Hedging Instruments (Details-4) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Current and Long-Term Regulatory Asset [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	$ 41,958	$ 49,281
Unrealized loss (gain)	128	(689)
Realized (loss) gain reclassified to a deferred account	(158)	(78)
Realized (loss) gain reclassified to income statement	(687)	(441)
Ending balance,	41,241	48,073
Other Current Assets [Member]
Change In Derivative Fair Value [Roll Forward]
Beginning balance,	806	230
Unrealized loss (gain)	0	0
Realized (loss) gain reclassified to a deferred account	158	78
Realized (loss) gain reclassified to income statement	(645)	(166)
Ending balance,	$ 319	$ 142