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Revenue: Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 344,312 $ 323,745 $ 1,198,714 $ 1,251,461
Total revenues 346,590 325,548 1,210,554 1,257,246
Services transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 6,568 6,846 20,062 19,934
Services transferred over time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 337,744 316,899 1,178,652 1,231,527
Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 2,278 1,803 11,840 5,785
Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 270,440 258,870 993,219 1,043,058
Transportation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 38,057 28,824 112,680 101,256
Wholesale        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 7,453 9,915 19,572 29,061
Market - off-system sales        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 7,667 5,103 12,014 12,062
Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 20,695 21,033 61,229 66,024
Inter-company Revenues        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (39,899) (28,449) (117,083) (82,796)
Total revenues (39,935) (37,943) (117,902) (111,502)
Inter-company Revenues | Services transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (8,100) (8,146) (23,855) (23,315)
Inter-company Revenues | Services transferred over time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (31,799) (20,303) (93,228) (59,481)
Inter-company Revenues | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues (36) (9,494) (819) (28,706)
Inter-company Revenues | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (8,100) (8,146) (23,855) (23,315)
Inter-company Revenues | Transportation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (139) (195) (416) (903)
Inter-company Revenues | Wholesale        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (24,521) (14,414) (72,609) (40,923)
Inter-company Revenues | Market - off-system sales        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (1,904) (1,488) (6,123) (5,047)
Inter-company Revenues | Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (5,235) (4,206) (14,080) (12,608)
Electric Utilities        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 200,618 191,150 536,107 538,200
Total revenues 200,842 191,384 538,181 540,665
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 200,618 191,150 536,107 538,200
Electric Utilities | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 224 234 2,074 2,465
Electric Utilities | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 169,505 162,214 459,949 455,409
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 5,925 8,210 14,947 23,334
Electric Utilities | Market - off-system sales        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9,535 6,452 17,940 16,592
Electric Utilities | Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 15,653 14,274 43,271 42,865
Gas Utilities        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 142,876 129,933 658,539 706,158
Total revenues 143,929 130,744 665,812 707,293
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 142,876 129,933 658,539 706,158
Gas Utilities | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 1,053 811 7,273 1,135
Gas Utilities | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 94,367 89,810 513,208 567,715
Gas Utilities | Transportation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 38,196 29,019 113,096 102,159
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 36 139 197 517
Gas Utilities | Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10,277 10,965 32,038 35,767
Power Generation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 26,049 16,119 77,234 46,650
Total revenues 26,518 25,811 78,606 75,764
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 26,049 16,119 77,234 46,650
Power Generation | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 469 9,692 1,372 29,114
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 26,049 16,119 77,234 46,650
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 14,668 14,992 43,917 43,249
Total revenues 15,236 15,552 45,857 45,026
Mining | Services transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 14,668 14,992 43,917 43,249
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 568 560 1,940 1,777
Mining | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 14,668 14,992 43,917 43,249
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