Variable Interest Entities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Variable Interest Entity [Line Items]
Purchased electricity	$ 622,288	$ 771,405	$ 773,429
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Variable Interest Entity [Line Items]
Purchased electricity		$ 175,900	$ 190,300