Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 231,060	$ 226,475	$ 682,650	$ 644,447
Net income (loss)	22,013	22,200	52,242	42,820
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	216,634	214,508	592,980	575,810
Net income (loss)	22,859	21,743	46,530	39,118
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	15,228	12,063	90,727	68,649
Net income (loss)	(1,726)	(1,461)	4,100	992
All Other
Segment Reporting Information [Line Items]
Operating revenues	249	307	742	851
Net income (loss)	880	1,918	1,612	2,710
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	231,060	226,475	682,650	644,447
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	216,545	214,426	592,730	575,532
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	14,266	11,742	89,178	68,064
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	249	307	742	851
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(1,051)	(403)	(1,799)	(863)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	89	82	250	278
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	962	321	1,549	585
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0