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Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 324,193	$ 329,746	$ 854,402	$ 927,716
Total revenues	(326,914)	(333,888)	(863,964)	(931,698)
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,930	5,387	13,494	13,088
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	317,263	324,359	840,908	914,628
Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(2,721)	(4,142)	(9,562)	(3,982)
Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	269,328	268,749	722,779	784,188
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	30,654	28,347	74,623	72,432
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4,712	8,547	12,119	19,146
Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,981	2,274	4,347	6,959
Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17,518	21,829	40,534	44,991
Inter-company Revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(38,542)	(26,147)	(77,184)	(54,347)
Total revenues	38,934	35,688	77,967	73,559
Inter-company Revenues | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(7,916)	(7,041)	(15,755)	(15,169)
Inter-company Revenues | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(30,626)	(19,106)	(61,429)	(39,178)
Inter-company Revenues | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	392	9,541	783	19,212
Inter-company Revenues | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(7,916)	(7,041)	(15,755)	(15,169)
Inter-company Revenues | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(138)	(276)	(277)	(708)
Inter-company Revenues | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(24,476)	(13,296)	(48,088)	(26,509)
Inter-company Revenues | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(1,580)	(1,335)	(4,219)	(3,559)
Inter-company Revenues | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(4,432)	(4,199)	(8,845)	(8,402)
Electric Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	161,573	164,377	335,489	347,050
Total revenues	(163,200)	(166,354)	(337,339)	(349,281)
Electric Utilities | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Electric Utilities | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	161,573	164,377	335,489	347,050
Electric Utilities | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(1,627)	(1,977)	(1,850)	(2,231)
Electric Utilities | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	141,804	139,732	290,444	293,195
Electric Utilities | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Electric Utilities | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,470	6,781	9,022	15,124
Electric Utilities | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,538	3,448	8,405	10,140
Electric Utilities | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12,761	14,416	27,618	28,591
Gas Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	160,598	164,026	515,663	576,225
Total revenues	(161,110)	(165,469)	(521,883)	(576,549)
Gas Utilities | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Gas Utilities | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	160,598	164,026	515,663	576,225
Gas Utilities | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(512)	(1,443)	(6,220)	(324)
Gas Utilities | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	120,594	123,630	418,841	477,905
Gas Utilities | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	30,792	28,623	74,900	73,140
Gas Utilities | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Gas Utilities | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	23	161	161	378
Gas Utilities | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9,189	11,612	21,761	24,802
Power Generation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25,718	15,062	51,185	30,531
Total revenues	(26,122)	(24,708)	(52,088)	(49,953)
Power Generation | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Power Generation | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25,718	15,062	51,185	30,531
Power Generation | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(404)	(9,646)	(903)	(19,422)
Power Generation | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Power Generation | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Power Generation | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25,718	15,062	51,185	30,531
Power Generation | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Power Generation | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mining
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,846	12,428	29,249	28,257
Total revenues	(15,416)	(13,045)	(30,621)	(29,474)
Mining | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,846	12,428	29,249	28,257
Mining | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mining | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(570)	(617)	(1,372)	(1,217)
Mining | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,846	12,428	29,249	28,257
Mining | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mining | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mining | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Mining | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 0	$ 0	$ 0	$ 0

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