Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 285,142,000		$ 241,415,000
Net income (loss)	24,235,000		19,685,000
Related Party Transaction, Electric Domestic Regulated Revenue	31,000,000		32,000,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	211,189,000		190,889,000
Net income (loss)	15,066,000		13,603,000
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	76,708,000		50,665,000
Net income (loss)	7,709,000		5,624,000
All Other
Segment Reporting Information [Line Items]
Operating revenues	254,000		247,000
Net income (loss)	1,460,000		458,000
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	285,142,000	[1]	241,415,000	[1]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	211,097,000	[1]	190,811,000	[1]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	73,791,000		50,357,000
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	254,000		247,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(3,009,000)		(386,000)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	92,000		78,000
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	2,917,000		308,000
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0

[1] Operating revenues include $31 million and $32 million of intercompany electric revenue for the three months ended March 31, 2014 and 2013, respectively.