Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,223,000	$ 1,073,300
Operating revenues	1,223,000	1,073,300
Net income	138,800	133,700
Related Party Transaction, Electric Domestic Regulated Revenue	1,100	1,100
Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	741,600	698,400
Operating revenues	741,500	698,300
Net income	82,400	79,600
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	469,200	364,100
Operating revenues	469,100	364,000
Net income	59,200	53,700
All Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	12,400	11,000
Operating revenues	12,400	11,000
Net income	(2,800)	400
Total revenues
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,223,200	1,073,500
Total revenues | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	741,500	698,300
Total revenues | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	469,100	364,000
Total revenues | All Other
Segment Reporting Information [Line Items]
Operating revenues	12,400	11,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	(200)	(200)
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	100	100
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0