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Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 630,217	$ 530,209
Total revenues	633,432	537,050
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,976	6,564
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	623,241	523,645
Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	3,215	6,841
Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	547,081	453,451
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	47,841	43,969
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10,163	7,407
Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4,845	2,366
Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20,287	23,016
Inter-company Revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(39,848)	(38,642)
Total revenues	(40,330)	(39,033)
Inter-company Revenues | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(7,107)	(7,839)
Inter-company Revenues | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(32,741)	(30,803)
Inter-company Revenues | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(482)	(391)
Inter-company Revenues | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(7,107)	(7,839)
Inter-company Revenues | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(110)	(139)
Inter-company Revenues | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(24,451)	(23,612)
Inter-company Revenues | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(2,884)	(2,639)
Inter-company Revenues | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(5,296)	(4,413)
Electric Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	227,271	173,916
Total revenues	227,408	174,139
Electric Utilities | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Electric Utilities | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	227,271	173,916
Electric Utilities | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	137	223
Electric Utilities | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	198,500	148,640
Electric Utilities | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Electric Utilities | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5,922	5,552
Electric Utilities | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7,656	4,867
Electric Utilities | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	15,193	14,857
Gas Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	400,019	355,065
Total revenues	402,519	360,773
Gas Utilities | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Gas Utilities | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	400,019	355,065
Gas Utilities | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	2,500	5,708
Gas Utilities | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	341,605	298,247
Gas Utilities | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	47,951	44,108
Gas Utilities | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Gas Utilities | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	73	138
Gas Utilities | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10,390	12,572
Power Generation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	28,692	25,467
Total revenues	29,163	25,966
Power Generation | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Power Generation | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	28,692	25,467
Power Generation | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	471	499
Power Generation | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Power Generation | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Power Generation | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	28,692	25,467
Power Generation | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Power Generation | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mining
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,083	14,403
Total revenues	14,672	15,205
Mining | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,083	14,403
Mining | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mining | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	589	802
Mining | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,083	14,403
Mining | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mining | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mining | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Mining | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 0	$ 0

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