Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,223,000	$ 1,073,300
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,182,400	1,010,600
Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,068,500	906,700
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	559,000	458,100
Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	497,000	436,400
Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	12,500	12,200
Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	57,300	47,900
Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	13,400	12,300
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	43,200	43,700
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	40,600	62,700
Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	741,500	698,300
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	707,800	662,000
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	625,500	583,000
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	245,100	227,600
Regulated Electric | Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	367,900	343,200
Regulated Electric | Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	12,500	12,200
Regulated Electric | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	57,300	47,900
Regulated Electric | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	13,400	12,300
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	11,600	18,800
Regulated Electric | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	33,700	36,300
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	469,100	364,000
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	463,300	338,700
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	431,700	313,800
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	311,400	227,800
Regulated Natural Gas | Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	120,300	86,000
Regulated Natural Gas | Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Regulated Natural Gas | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Regulated Natural Gas | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	31,600	24,900
Regulated Natural Gas | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	5,800	25,300
All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	12,400	11,000
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	11,300	9,900
All Other | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	11,300	9,900
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,500	2,700
All Other | Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	8,800	7,200
All Other | Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
All Other | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
All Other | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
All Other | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Total operating revenues	1,100	1,100
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 12,400	$ 11,000