Derivative and Hedging Instruments (Details-2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Regulatory Asset [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	$ 71,514	$ 21,677	$ 42,356	$ 19,230
Change in unrealized loss (gain)	9,791	888	43,101	3,936
Realized loss reclassified to a deferred account	(222)	(610)	(2,880)	(1,274)
Realized gain (loss) reclassified to income statement	(1,721)	649	(3,215)	712
Ending balance,	79,362	22,604	79,362	22,604
Other Current Assets [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	672	505	1,604	1,411
Change in unrealized loss (gain)	0	0	0	0
Realized loss reclassified to a deferred account	222	610	2,880	1,274
Realized gain (loss) reclassified to income statement	(176)	(114)	(3,766)	(1,684)
Ending balance,	$ 718	$ 1,001	$ 718	$ 1,001