Revenues (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 1,010,642	$ 1,028,365
Alternative revenue and other		62,656	52,169
Total operating revenues		$ 1,073,298	1,080,534
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs		24 months
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1],[2]	$ 1,073,298	1,080,534
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		906,740	930,845
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		458,091	462,908
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		436,413	455,199
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		12,236	12,738
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		47,890	43,576
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		12,252	14,639
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		43,760	39,305
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		662,024	681,716
Alternative revenue and other		36,250	29,672
Total operating revenues		698,386	711,480
Regulated Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	698,274	711,388
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		583,051	606,806
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		227,649	234,434
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		343,226	359,697
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		12,176	12,675
Regulated Electric | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		47,890	43,576
Regulated Electric | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		12,252	14,639
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		18,831	16,695
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		338,703	334,762
Alternative revenue and other		25,283	21,374
Total operating revenues		364,050	356,192
Regulated Natural Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues		363,986	356,136
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		313,774	312,152
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		227,746	225,910
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		86,028	86,242
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
Regulated Natural Gas | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
Regulated Natural Gas | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		24,929	22,610
All Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		9,915	11,887
Alternative revenue and other		1,123	1,123
Total operating revenues		11,038	13,010
All Other | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	[2]	11,038	13,010
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		9,915	11,887
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		2,696	2,564
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		7,159	9,260
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		60	63
All Other | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
All Other | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		$ 0	$ 0

[1] Operating revenues include $1 million and $2 million of affiliate electric revenue for the three months ended March 31, 2018 and 2017.

[2] Operating revenues include $1 million of other affiliate revenue for the three months ended March 31, 2018 and 2017.