Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 210,175	$ 194,173	$ 451,590	$ 417,972
Net income	10,544	5,742	30,229	20,620
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	185,374	179,105	376,185	361,106
Net income	10,068	6,127	23,671	17,375
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	24,555	14,830	74,912	56,322
Net income	201	(947)	5,825	2,453
All Other
Segment Reporting Information [Line Items]
Operating revenues	246	238	493	544
Net income	275	562	733	792
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	210,175	194,173	451,590	417,972
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	185,457	179,214	376,346	361,302
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	24,833	14,891	75,498	56,586
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	246	238	493	544
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(361)	(170)	(747)	(460)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	83	109	161	196
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	278	61	586	264
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0