Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of reportable segments			1
Net income	$ 30,294,000	$ 24,672,000	$ 41,654,000	$ 34,910,000
Total assets	3,381,819,000		3,381,819,000		3,223,849,000
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	380,200,000	433,300,000	720,700,000	814,500,000
Net income	30,300,000	24,700,000	41,700,000	34,900,000
Capital expenditures			185,900,000	158,700,000
Total assets	$ 3,381,819,000		$ 3,381,819,000		$ 3,223,849,000