Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 530,209	$ 597,970
Total revenues	537,050	597,810
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,564	7,701
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	523,645	590,269
Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	6,841	(160)
Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	453,451	515,439
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	43,969	44,085
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7,407	10,599
Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,366	4,685
Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	23,016	23,162
Inter-company Revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(38,642)	(36,878)
Total revenues	(39,033)	(37,871)
Inter-company Revenues | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(7,839)	(8,128)
Inter-company Revenues | Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(30,803)	(28,750)
Inter-company Revenues | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	(391)	(993)
Inter-company Revenues | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(7,839)	(8,128)
Inter-company Revenues | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(139)	(432)
Inter-company Revenues | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(23,612)	(21,891)
Inter-company Revenues | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(2,639)	(2,224)
Inter-company Revenues | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(4,413)	(4,203)
Electric Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	173,916	182,673
Total revenues	174,139	182,927
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	173,916	182,673
Electric Utilities | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	223	254
Electric Utilities | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	148,640	153,463
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	5,552	8,343
Electric Utilities | Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4,867	6,692
Electric Utilities | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,857	14,175
Gas Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	355,065	412,199
Total revenues	360,773	411,080
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	355,065	412,199
Gas Utilities | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	5,708	(1,119)
Gas Utilities | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	298,247	354,275
Gas Utilities | Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44,108	44,517
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	138	217
Gas Utilities | Transmission/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12,572	13,190
Power Generation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25,467	24,147
Total revenues	25,966	25,245
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	25,467	24,147
Power Generation | Other revenues
Disaggregation of Revenue [Line Items]
Total revenues	499	1,098
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	25,467	24,147
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	14,403	15,829
Total revenues	15,205	16,429
Mining | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,403	15,829
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	802	600
Mining | Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14,403	15,829
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