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Business Segment Information: Information Relating to Segment Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information
Revenue from contracts with customers	$ 344,312			$ 323,745			$ 1,198,714	$ 1,251,461
Revenues	346,590			325,548			1,210,554	1,257,246
Operating income	82,234			70,551			302,352	284,683
Interest expense, net	(36,041)			(33,487)			(107,039)	(102,469)
Impairment of investment	0		$ (6,900)	(19,741)			(6,859)	(19,741)
Other income (expense), net	(1,193)			580			(703)	55
Income tax (expense)	(4,651)			(2,508)			(25,484)	(22,078)
Net income	40,349	$ 24,694	$ 97,224	15,395	$ 17,693	$ 107,362	162,267	140,450
Net income attributable to noncontrolling interest	(4,066)			(3,655)			(11,844)	(10,319)
Net income available for common stock	36,283			11,740			150,423	130,131
Inter-company Revenues
Segment Reporting Information
Revenue from contracts with customers	(39,899)			(28,449)			(117,083)	(82,796)
Revenues	(39,935)			(37,943)			(117,902)	(111,502)
Consolidation, Eliminations
Segment Reporting Information
Revenue from contracts with customers	0			0			0	0
Corporate and Other
Segment Reporting Information
Operating income	(239)			(34)			(108)	(439)
Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	200,618			191,150			536,107	538,200
Revenues	200,842			191,384			538,181	540,665
Operating income	52,083			50,653			121,726	125,219
Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	142,876			129,933			658,539	706,158
Revenues	143,929			130,744			665,812	707,293
Operating income	18,147			4,736			139,253	116,607
Power Generation
Segment Reporting Information
Revenue from contracts with customers	26,049			16,119			77,234	46,650
Revenues	26,518			25,811			78,606	75,764
Operating income	8,738			11,822			31,489	33,945
Mining
Segment Reporting Information
Revenue from contracts with customers	14,668			14,992			43,917	43,249
Revenues	15,236			15,552			45,857	45,026
Operating income	3,505			3,374			9,992	9,351
Other Revenues
Segment Reporting Information
Revenues	2,278			1,803			11,840	5,785
Other Revenues | Inter-company Revenues
Segment Reporting Information
Revenues	(36)			(9,494)			(819)	(28,706)
Other Revenues | Consolidation, Eliminations
Segment Reporting Information
Revenues	0			0			0	0
Other Revenues | Electric Utilities
Segment Reporting Information
Revenues	224			234			2,074	2,465
Other Revenues | Gas Utilities
Segment Reporting Information
Revenues	1,053			811			7,273	1,135
Other Revenues | Power Generation
Segment Reporting Information
Revenues	469			9,692			1,372	29,114
Other Revenues | Mining
Segment Reporting Information
Revenues	568			560			1,940	1,777
External Operating Revenue
Segment Reporting Information
Revenue from contracts with customers	344,312			323,745			1,198,714	1,251,461
External Operating Revenue | Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	194,941			185,811			518,641	521,614
External Operating Revenue | Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	141,275			129,385			655,386	704,188
External Operating Revenue | Power Generation
Segment Reporting Information
Revenue from contracts with customers	1,528			1,703			4,625	5,725
External Operating Revenue | Mining
Segment Reporting Information
Revenue from contracts with customers	6,568			6,846			20,062	19,934
External Operating Revenue | Other Revenues
Segment Reporting Information
Revenues	2,278			1,803			11,840	5,785
External Operating Revenue | Other Revenues | Electric Utilities
Segment Reporting Information
Revenues	224			234			2,074	2,465
External Operating Revenue | Other Revenues | Gas Utilities
Segment Reporting Information
Revenues	863			810			7,083	1,134
External Operating Revenue | Other Revenues | Power Generation
Segment Reporting Information
Revenues	414			531			1,206	1,401
External Operating Revenue | Other Revenues | Mining
Segment Reporting Information
Revenues	777			228			1,477	785
Inter-company Operating Revenue | Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	5,677			5,339			17,466	16,586
Inter-company Operating Revenue | Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	1,601			549			3,153	1,971
Inter-company Operating Revenue | Power Generation
Segment Reporting Information
Revenue from contracts with customers	24,521			14,415			72,609	40,924
Inter-company Operating Revenue | Mining
Segment Reporting Information
Revenue from contracts with customers	8,100			8,146			23,855	23,315
Inter-company Operating Revenue | Other Revenues | Electric Utilities
Segment Reporting Information
Revenues	0			0			0	0
Inter-company Operating Revenue | Other Revenues | Gas Utilities
Segment Reporting Information
Revenues	190			0			190	0
Inter-company Operating Revenue | Other Revenues | Power Generation
Segment Reporting Information
Revenues	55			9,162			166	27,714
Inter-company Operating Revenue | Other Revenues | Mining
Segment Reporting Information
Revenues	$ (209)			$ 332			$ 463	$ 992

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