Derivative and Hedging Instruments (Details-2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Regulatory Asset [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	$ 79,362	$ 22,604	$ 42,356	$ 19,230
Change in unrealized loss (gain)	(468)	3,839	42,633	7,775
Realized loss reclassified to a deferred account	(223)	(210)	(3,103)	(1,484)
Realized gain (loss) reclassified to income statement	(3,414)	350	(6,629)	1,062
Ending balance,	75,257	26,583	75,257	26,583
Other Current Assets [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	718	1,001	1,604	1,411
Change in unrealized loss (gain)	0	0	0	0
Realized loss reclassified to a deferred account	223	210	3,103	1,484
Realized gain (loss) reclassified to income statement	(161)	(233)	(3,927)	(1,917)
Ending balance,	$ 780	$ 978	$ 780	$ 978