Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues		$ 264,931	$ 254,850
Net income (loss)		22,419	17,631
Affiliate electric revenue		42,000	41,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		216,405	211,633
Net income (loss)		15,470	11,501
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		48,461	43,100
Net income (loss)		6,545	6,092
All Other
Segment Reporting Information [Line Items]
Operating revenues		275	306
Net income (loss)		404	38
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]	264,931	254,850
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	216,309	211,524
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		48,347	43,020
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues		275	306
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(210)	(189)
Net income (loss)		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		96	109
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		114	80
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0

[1] (a) Operating revenues include $42 million and $41 million of affiliate electric revenue for the three months ended March 31, 2017 and 2016, respectively.