Derivative and Hedging Instruments (Details-2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Regulatory Asset [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	$ 42,356	$ 19,230
Change in unrealized loss (gain)	33,310	3,047
Realized loss reclassified to a deferred account	(2,658)	(663)
Realized gain (loss) reclassified to income statement	(1,494)	63
Ending balance,	71,514	21,677
Other Current Assets [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	1,604	1,411
Change in unrealized loss (gain)	0	0
Realized loss reclassified to a deferred account	2,658	663
Realized gain (loss) reclassified to income statement	(3,590)	(1,569)
Ending balance,	$ 672	$ 505