Segment Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of reportable segments			1
Net income	$ 58,247,000	$ 48,581,000	$ 99,901,000	$ 83,491,000
Total assets	3,465,873,000		3,465,873,000		3,223,849,000
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	471,900,000	522,900,000	1,192,600,000	1,337,400,000
Net income	58,200,000	48,600,000	99,900,000	83,500,000
Capital expenditures			280,600,000	222,300,000
Total assets	$ 3,500,000,000		$ 3,500,000,000		$ 3,200,000,000