Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Operating revenues	$ 540,063	$ 551,623	$ 1,468,633	$ 1,491,491
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs			24 months
Regulated Electric [Member] | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	537,294	544,714	$ 1,453,874	1,454,735
Regulated Electric [Member] | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	356,827	368,074	939,541	970,479
Regulated Electric [Member] | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	114,387	113,380	279,543	277,169
Regulated Electric [Member] | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	229,457	241,295	625,988	658,057
Regulated Electric [Member] | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	12,983	13,399	34,010	35,253
Regulated Electric [Member] | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	117,949	116,635	326,810	309,669
Regulated Electric [Member] | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	60,726	56,143	175,342	166,715
Regulated Electric [Member] | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,792	3,862	12,181	7,872
Regulated Electric [Member] | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	2,769	6,909	14,759	36,756
Total revenues | Regulated Electric [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	$ 540,063	$ 551,623	$ 1,468,633	$ 1,491,491