Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 273,144	$ 264,931
Net income (loss)		31,417	22,419
Affiliate electric revenue		38,000	42,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		216,475	216,405
Net income (loss)		20,444	15,470
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		56,648	48,461
Net income (loss)		10,399	6,545
All Other
Segment Reporting Information [Line Items]
Operating revenues		278	275
Net income (loss)		574	404
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]	273,144	264,931
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	216,366	216,309
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		56,500	48,347
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues		278	275
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(257)	(210)
Net income (loss)		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		109	96
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		148	114
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0

[1] (a) Operating revenues include $38 million and $42 million of affiliate electric revenue for the three months ended March 31, 2018 and 2017, respectively.