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Revenue: Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 378,653 $ 344,312 $ 1,377,293 $ 1,198,714
Total revenues 380,590 346,590 1,386,594 1,210,554
Services transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 6,968 6,568 20,658 20,062
Services transferred over time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 371,685 337,744 1,356,635 1,178,652
Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 1,937 2,278 9,301 11,840
Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 302,858 270,440 1,158,968 993,219
Transportation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 37,541 38,057 116,922 112,680
Wholesale        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 6,189 7,453 20,794 19,572
Market - off-system sales        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 13,586 7,667 25,784 12,014
Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 18,479 20,695 54,825 61,229
Inter-company Revenues        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (39,616) (39,899) (117,167) (117,083)
Total revenues (40,104) (39,935) (118,622) (117,902)
Inter-company Revenues | Services transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (8,846) (8,100) (23,093) (23,855)
Inter-company Revenues | Services transferred over time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (30,770) (31,799) (94,074) (93,228)
Inter-company Revenues | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues (488) (36) (1,455) (819)
Inter-company Revenues | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (8,846) (8,100) (23,093) (23,855)
Inter-company Revenues | Transportation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (110) (139) (329) (416)
Inter-company Revenues | Wholesale        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (23,725) (24,521) (71,656) (72,609)
Inter-company Revenues | Market - off-system sales        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (1,638) (1,904) (6,197) (6,123)
Inter-company Revenues | Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers (5,297) (5,235) (15,892) (14,080)
Electric Utilities        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 212,900 200,618 622,825 536,107
Total revenues 213,103 200,842 625,444 538,181
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 212,900 200,618 622,825 536,107
Electric Utilities | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 203 224 2,619 2,074
Electric Utilities | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 179,982 169,505 536,952 459,949
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 3,856 5,925 12,788 14,947
Electric Utilities | Market - off-system sales        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 15,149 9,535 31,746 17,940
Electric Utilities | Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 13,913 15,653 41,339 43,271
Gas Utilities        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 163,497 142,876 748,222 658,539
Total revenues 164,683 143,929 753,252 665,812
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 163,497 142,876 748,222 658,539
Gas Utilities | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 1,186 1,053 5,030 7,273
Gas Utilities | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 115,908 94,367 601,358 513,208
Gas Utilities | Transportation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 37,651 38,196 117,251 113,096
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 75 36 235 197
Gas Utilities | Transmission/Other        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9,863 10,277 29,378 32,038
Power Generation        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 26,058 26,049 79,662 77,234
Total revenues 26,520 26,518 81,031 78,606
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 26,058 26,049 79,662 77,234
Power Generation | Other revenues        
Disaggregation of Revenue [Line Items]        
Total revenues 462 469 1,369 1,372
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 26,058 26,049 79,662 77,234
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 15,814 14,668 43,751 43,917
Total revenues 16,388 15,236 45,489 45,857
Mining | Services transferred at a point in time        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 15,814 14,668 43,751 43,917
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 574 568 1,738 1,940
Mining | Retail - Electric , Natural Gas and Coal        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 15,814 14,668 43,751 43,917
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