Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 Segment	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of reportable segments	1
Net income	$ 12,584,000	$ 11,360,000
Assets	3,599,403,000		3,517,941,000
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	374,257,000	340,488,000
Net income	12,584,000	11,360,000
Capital expenditures	99,200,000	82,500,000
Assets	$ 3,599,403,000		$ 3,517,941,000