Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues		$ 254,850	$ 273,960
Net income (loss)		17,631	22,267
Affiliate electric revenue		41,000	38,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		211,633	210,393
Net income (loss)		11,501	15,514	[1]
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		43,100	63,521
Net income (loss)		6,092	6,781
All Other
Segment Reporting Information [Line Items]
Operating revenues		306	380
Net income (loss)		38	(28)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[2]	254,850	273,960
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[2]	211,524	210,284
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		43,020	63,296
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues		306	380
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(189)	(334)
Net income (loss)		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		109	109
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		80	225
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0

[1] (b) Includes a net of tax charge related to the Monticello LCM/EPU project. See Note 5.

[2] (a) Operating revenues include $41 million and $38 million of affiliate electric revenue for the three months ended March 31, 2016 and 2015, respectively.