Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Related Party Transaction - Electric and Gas Domestic Regulated Revenue	$ 44,100	$ 37,700
Revenue from Contract with Customer, Excluding Assessed Tax	276,200	273,100
Operating revenues	276,200	273,100
Net income (loss)	24,000	31,400
Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	215,100	216,400
Net income (loss)	13,200	20,400
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	61,300	56,600
Net income (loss)	10,500	10,400
All Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	100	300
Net income (loss)	300	600
Total revenues
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	276,500	273,300
Operating revenues	276,200	273,100
Total revenues | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	215,000	216,300
Operating revenues	215,000	216,300
Total revenues | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	61,100	56,500
Operating revenues	61,100	56,500
Total revenues | All Other
Segment Reporting Information [Line Items]
Operating revenues	100	300
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(300)	(200)
Net income (loss)	24,000	31,400
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	100	100
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	200	100
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0