Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net income	$ 28,206,000	$ 30,294,000	$ 40,790,000	$ 41,654,000
Assets	3,763,982,000		3,763,982,000		3,517,941,000
Regulated Electric
Segment Reporting Information [Line Items]
Number of reportable segments			1
Operating revenues	461,800,000	380,200,000	836,100,000	720,700,000
Net income	28,200,000	30,300,000	40,800,000	41,700,000
Capital expenditures			254,300,000	184,900,000
Assets	$ 3,800,000,000		$ 3,800,000,000		$ 3,500,000,000