Derivative and Hedging Instruments (Details-2) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Regulatory Asset [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	$ 21,677	$ 19,891	$ 19,230	$ 12,542
Change in unrealized loss (gain)	888	(2,037)	3,936	6,677
Realized loss reclassified to a deferred account	(610)	(104)	(1,274)	(810)
Realized gain (loss) reclassified to income statement	649	(405)	712	(1,064)
Balance at June 30,	22,604	17,345	22,604	17,345
Other Current Assets [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	505	1,119	1,411	1,334
Change in unrealized loss (gain)	0	0	0	0
Realized loss reclassified to a deferred account	610	104	1,274	810
Realized gain (loss) reclassified to income statement	(114)	(193)	(1,684)	(1,114)
Balance at June 30,	$ 1,001	$ 1,030	$ 1,001	$ 1,030