Derivative and Hedging Instruments (Details-3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Regulatory Asset [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	$ 72,329	$ 42,356
Change in unrealized loss (gain)	(1,697)	33,310
Realized loss reclassified to a deferred account	(290)	(2,658)
Realized gain (loss) reclassified to income statement	(1,145)	(1,494)
Ending balance,	69,197	71,514
Other Current Assets [Member]
Unrealized and realized gains (losses) on balance sheet [Line Items]
Beginning balance,	574	1,604
Change in unrealized loss (gain)	0	0
Realized loss reclassified to a deferred account	290	2,658
Realized gain (loss) reclassified to income statement	(605)	(3,590)
Ending balance,	$ 259	$ 672