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Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from contracts with customers	$ 1,680,804	$ 1,726,392	$ 1,748,542
Revenue	1,696,941	1,734,900	1,754,268
Services transferred at a point in time
Revenue from contracts with customers	27,089	27,180	33,609
Services transferred over time
Revenue from contracts with customers	1,653,715	1,699,212	1,714,933
Other revenues
Revenue	16,137	8,508	5,726
Intercompany Eliminations
Revenue from contracts with customers	(157,285)	(149,205)	(141,428)
Revenue	(157,895)	(150,769)	(142,974)
Intercompany Eliminations | Services transferred at a point in time
Revenue from contracts with customers	(31,478)	(32,053)	(32,194)
Intercompany Eliminations | Services transferred over time
Revenue from contracts with customers	(125,807)	(117,152)	(109,234)
Intercompany Eliminations | Other revenues
Revenue	(610)	(1,564)	(1,546)
Electric Utilities
Revenue from contracts with customers	711,843	707,561	709,024
Revenue	714,044	712,752	711,451
Electric Utilities | Services transferred at a point in time
Revenue from contracts with customers	0	0	0
Electric Utilities | Services transferred over time
Revenue from contracts with customers	711,843	707,561	709,024
Electric Utilities | Other revenues
Revenue	2,201	5,191	2,427
Gas Utilities
Revenue from contracts with customers	964,421	1,009,646	1,024,352
Revenue	974,670	1,010,030	1,025,307
Gas Utilities | Services transferred at a point in time
Revenue from contracts with customers	0	0	0
Gas Utilities | Services transferred over time
Revenue from contracts with customers	964,421	1,009,646	1,024,352
Gas Utilities | Other revenues
Revenue	10,249	384	955
Power Generation
Revenue from contracts with customers	103,258	99,157	90,791
Revenue	105,047	101,258	92,451
Power Generation | Services transferred at a point in time
Revenue from contracts with customers	0	0	0
Power Generation | Services transferred over time
Revenue from contracts with customers	103,258	99,157	90,791
Power Generation | Other revenues
Revenue	1,789	2,101	1,660
Mining
Revenue from contracts with customers	58,567	59,233	65,803
Revenue	61,075	61,629	68,033
Mining | Services transferred at a point in time
Revenue from contracts with customers	58,567	59,233	65,803
Mining | Services transferred over time
Revenue from contracts with customers	0	0	0
Mining | Other revenues
Revenue	2,508	2,396	2,230
Retail
Revenue from contracts with customers	1,403,732	1,450,776	1,461,317
Retail | Intercompany Eliminations
Revenue from contracts with customers	(31,478)	(32,053)	(32,194)
Retail | Electric Utilities
Revenue from contracts with customers	610,721	605,756	594,329
Retail | Gas Utilities
Revenue from contracts with customers	765,922	817,840	833,379
Retail | Power Generation
Revenue from contracts with customers	0	0	0
Retail | Mining
Revenue from contracts with customers	58,567	59,233	65,803
Transportation
Revenue from contracts with customers	154,055	142,348	139,357
Transportation | Intercompany Eliminations
Revenue from contracts with customers	(526)	(1,042)	(1,348)
Transportation | Electric Utilities
Revenue from contracts with customers	0	0	0
Transportation | Gas Utilities
Revenue from contracts with customers	154,581	143,390	140,705
Transportation | Power Generation
Revenue from contracts with customers	0	0	0
Transportation | Mining
Revenue from contracts with customers	0	0	0
Wholesale
Revenue from contracts with customers	23,937	28,464	39,521
Wholesale | Intercompany Eliminations
Revenue from contracts with customers	(97,169)	(91,577)	(84,957)
Wholesale | Electric Utilities
Revenue from contracts with customers	17,848	20,884	33,687
Wholesale | Gas Utilities
Revenue from contracts with customers	0	0	0
Wholesale | Power Generation
Revenue from contracts with customers	103,258	99,157	90,791
Wholesale | Mining
Revenue from contracts with customers	0	0	0
Market - off-system sales
Revenue from contracts with customers	15,772	16,772	17,563
Market - off-system sales | Intercompany Eliminations
Revenue from contracts with customers	(8,797)	(7,736)	(8,102)
Market - off-system sales | Electric Utilities
Revenue from contracts with customers	24,309	23,817	24,799
Market - off-system sales | Gas Utilities
Revenue from contracts with customers	260	691	866
Market - off-system sales | Power Generation
Revenue from contracts with customers	0	0	0
Market - off-system sales | Mining
Revenue from contracts with customers	0	0	0
Transmission/Other
Revenue from contracts with customers	83,308	88,032	90,784
Transmission/Other | Intercompany Eliminations
Revenue from contracts with customers	(19,315)	(16,797)	(14,827)
Transmission/Other | Electric Utilities
Revenue from contracts with customers	58,965	57,104	56,209
Transmission/Other | Gas Utilities
Revenue from contracts with customers	43,658	47,725	49,402
Transmission/Other | Power Generation
Revenue from contracts with customers	0	0	0
Transmission/Other | Mining
Revenue from contracts with customers	$ 0	$ 0	$ 0

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