Segments and Related Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 Segment	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Number of reportable segments									1
Operating revenues	$ 347,477,000	$ 471,889,000	$ 380,201,000	$ 340,488,000	$ 370,146,000	$ 522,921,000	$ 433,289,000	$ 381,209,000	$ 1,540,055,000	$ 1,707,565,000	$ 1,612,990,000
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									$ 1,540,100	$ 1,707,600	$ 1,613,000