Derivative and Hedging Instruments (Details-2) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Regulatory Asset [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	$ 22,604	$ 17,345	$ 19,230	$ 12,542
Change in unrealized loss (gain)	3,839	4,055	7,775	10,731
Realized loss reclassified to a deferred account	(210)	(572)	(1,484)	(1,381)
Realized gain (loss) reclassified to income statement	350	(177)	1,062	(1,241)
Ending balance,	26,583	20,651	26,583	20,651
Other Current Assets [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	1,001	1,030	1,411	1,334
Change in unrealized loss (gain)	0	0	0	0
Realized loss reclassified to a deferred account	210	572	1,484	1,381
Realized gain (loss) reclassified to income statement	(233)	(399)	(1,917)	(1,512)
Ending balance,	$ 978	$ 1,203	$ 978	$ 1,203