Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenue from contracts with customers	$ 400,162	$ 460,868	$ 1,717,353	$ 1,752,961
Revenue	407,126	462,612	1,739,568	1,760,377
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	311,832	368,692	1,453,433	1,500,617
Transportation
Revenue from contracts with customers	42,540	40,907	131,066	124,898
Wholesale
Revenue from contracts with customers	9,441	13,667	24,578	32,370
Market - off-system sales
Revenue from contracts with customers	13,024	16,956	37,836	33,192
Transmission/Other
Revenue from contracts with customers	23,325	20,646	70,440	61,884
Other revenues
Revenue	6,964	1,744	22,215	7,416
Services transferred at a point in time
Revenue from contracts with customers	7,843	7,928	24,344	21,712
Services transferred over time
Revenue from contracts with customers	392,319	452,940	1,693,009	1,731,249
Inter-segment Revenues
Revenue from contracts with customers	(4,488)	(4,247)	(13,464)	(12,743)
Revenue	(4,488)	(4,345)	(13,464)	(13,058)
Inter-segment Revenues \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	0	0	0	0
Inter-segment Revenues \| Transportation
Revenue from contracts with customers	(114)	(99)	(344)	(298)
Inter-segment Revenues \| Wholesale
Revenue from contracts with customers	0	0	0	0
Inter-segment Revenues \| Market - off-system sales
Revenue from contracts with customers	0	0	0	0
Inter-segment Revenues \| Transmission/Other
Revenue from contracts with customers	(4,374)	(4,148)	(13,120)	(12,445)
Inter-segment Revenues \| Other revenues
Revenue	0	(98)	0	(315)
Inter-segment Revenues \| Services transferred at a point in time
Revenue from contracts with customers	0	0	0	0
Inter-segment Revenues \| Services transferred over time
Revenue from contracts with customers	(4,488)	(4,247)	(13,464)	(12,743)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	231,561	257,845	639,096	664,822
Revenue	237,329	258,669	649,111	669,586
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	191,029	211,489	522,304	553,327
Operating Segments \| Electric Utilities \| Transportation
Revenue from contracts with customers	0	0	0	0
Operating Segments \| Electric Utilities \| Wholesale
Revenue from contracts with customers	9,441	13,667	24,578	32,370
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	12,999	16,770	37,487	32,590
Operating Segments \| Electric Utilities \| Transmission/Other
Revenue from contracts with customers	18,092	15,919	54,727	46,535
Operating Segments \| Electric Utilities \| Other revenues
Revenue	5,768	824	10,015	4,764
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	7,843	7,928	24,344	21,712
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	223,718	249,917	614,752	643,110
Operating Segments \| Gas Utilities
Revenue from contracts with customers	173,089	207,270	1,091,721	1,100,882
Revenue	174,285	208,288	1,103,921	1,103,849
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	120,803	157,203	931,129	947,290
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	42,654	41,006	131,410	125,196
Operating Segments \| Gas Utilities \| Wholesale
Revenue from contracts with customers	0	0	0	0
Operating Segments \| Gas Utilities \| Market - off-system sales
Revenue from contracts with customers	25	186	349	602
Operating Segments \| Gas Utilities \| Transmission/Other
Revenue from contracts with customers	9,607	8,875	28,833	27,794
Operating Segments \| Gas Utilities \| Other revenues
Revenue	1,196	1,018	12,200	2,967
Operating Segments \| Gas Utilities \| Services transferred at a point in time
Revenue from contracts with customers	0	0	0	0
Operating Segments \| Gas Utilities \| Services transferred over time
Revenue from contracts with customers	$ 173,089	$ 207,270	$ 1,091,721	$ 1,100,882