Segment Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net income	$ 35,037,000	$ 58,247,000	$ 75,827,000	$ 99,901,000
Assets	3,937,626,000		3,937,626,000		3,517,941,000
Regulated Electric
Segment Reporting Information [Line Items]
Number of reportable segments			1
Operating revenues	481,400,000	471,900,000	1,317,500,000	1,192,600,000
Net income	35,000,000	58,200,000	75,800,000	99,900,000
Capital expenditures			423,400,000	280,600,000
Assets	$ 3,900,000,000		$ 3,900,000,000		$ 3,500,000,000