Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 241,415	$ 223,799
Net income	19,685	14,878
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	190,811	182,001
Net income	13,603	11,248
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	50,357	41,492
Net income	5,624	3,400
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	247	306
Net income	458	230
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	241,415	223,799
Operating Segments [Member] | Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	190,889	182,088
Operating Segments [Member] | Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	50,665	41,695
Operating Segments [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	247	306
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(386)	(290)
Net income	0	0
Intersegment Eliminations [Member] | Regulated Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	78	87
Intersegment Eliminations [Member] | Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	308	203
Intersegment Eliminations [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0