Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues from external customers	$ 226,475	$ 224,468	$ 644,447	$ 661,426
Intersegment revenues	0	0	0	0
Total operating revenues	226,475	224,468	644,447	661,426
Net income (loss)	22,200	19,973	42,820	43,094
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	214,426	210,810	575,532	573,870
Intersegment revenues	82	83	278	299
Total operating revenues	214,508	210,893	575,810	574,169
Net income (loss)	21,743	21,178	39,118	40,698
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	11,742	13,312	68,064	86,675
Intersegment revenues	321	212	585	1,050
Total operating revenues	12,063	13,524	68,649	87,725
Net income (loss)	(1,461)	(1,835)	992	1,757
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	307	346	851	881
Intersegment revenues	0	0	0	0
Total operating revenues	307	346	851	881
Net income (loss)	1,918	630	2,710	639
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from external customers	0	0	0	0
Intersegment revenues	(403)	(295)	(863)	(1,349)
Total operating revenues	(403)	(295)	(863)	(1,349)
Net income (loss)	$ 0	$ 0	$ 0	$ 0