Business Segment Information: Information Relating to Segment Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Segment Reporting Information
Revenue from contracts with customers	$ 530,209		$ 597,970
Revenues	537,050		597,810
Operating income	153,185		160,131
Interest expense, net	(35,453)		(34,717)
Impairment of investment	(6,859)	$ (20,000)	0
Other Nonoperating Income (Expense) Including Allowance For Funds Used During Construction, Equity	2,353		(789)
Income tax (expense)	(16,002)		(17,263)
Net income	97,224		107,362
Net income attributable to noncontrolling interest	(4,050)		(3,554)
Net income available for common stock	93,174		103,808
Inter-company Revenues
Segment Reporting Information
Revenue from contracts with customers	(38,642)		(36,878)
Revenues	(39,033)		(37,871)
Consolidation, Eliminations
Segment Reporting Information
Revenue from contracts with customers	0		0
Corporate
Segment Reporting Information
Operating income	160		(507)
Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	173,916		182,673
Revenues	174,139		182,927
Operating income	35,650		41,020
Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	355,065		412,199
Revenues	360,773		411,080
Operating income	102,897		103,314
Power Generation
Segment Reporting Information
Revenue from contracts with customers	25,467		24,147
Revenues	25,966		25,245
Operating income	11,349		11,967
Mining
Segment Reporting Information
Revenue from contracts with customers	14,403		15,829
Revenues	15,205		16,429
Operating income	3,129		4,337
Other Revenues
Segment Reporting Information
Revenues	6,841		(160)
Other Revenues | Inter-company Revenues
Segment Reporting Information
Revenues	(391)		(993)
Other Revenues | Consolidation, Eliminations
Segment Reporting Information
Revenues	0		0
Other Revenues | Electric Utilities
Segment Reporting Information
Revenues	223		254
Other Revenues | Gas Utilities
Segment Reporting Information
Revenues	5,708		(1,119)
Other Revenues | Power Generation
Segment Reporting Information
Revenues	499		1,098
Other Revenues | Mining
Segment Reporting Information
Revenues	802		600
External Operating Revenue
Segment Reporting Information
Revenue from contracts with customers	530,209		597,970
External Operating Revenue | Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	167,503		176,663
External Operating Revenue | Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	354,287		411,500
External Operating Revenue | Power Generation
Segment Reporting Information
Revenue from contracts with customers	1,855		2,257
External Operating Revenue | Mining
Segment Reporting Information
Revenue from contracts with customers	6,564		7,550
External Operating Revenue | Other Revenues
Segment Reporting Information
Revenues	6,841		(160)
External Operating Revenue | Other Revenues | Electric Utilities
Segment Reporting Information
Revenues	223		254
External Operating Revenue | Other Revenues | Gas Utilities
Segment Reporting Information
Revenues	5,708		(1,119)
External Operating Revenue | Other Revenues | Power Generation
Segment Reporting Information
Revenues	443		436
External Operating Revenue | Other Revenues | Mining
Segment Reporting Information
Revenues	467		269
Inter-company Operating Revenue | Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	6,413		6,010
Inter-company Operating Revenue | Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	778		699
Inter-company Operating Revenue | Power Generation
Segment Reporting Information
Revenue from contracts with customers	23,612		21,890
Inter-company Operating Revenue | Mining
Segment Reporting Information
Revenue from contracts with customers	7,839		8,279
Inter-company Operating Revenue | Other Revenues | Electric Utilities
Segment Reporting Information
Revenues	0		0
Inter-company Operating Revenue | Other Revenues | Gas Utilities
Segment Reporting Information
Revenues	0		0
Inter-company Operating Revenue | Other Revenues | Power Generation
Segment Reporting Information
Revenues	56		662
Inter-company Operating Revenue | Other Revenues | Mining
Segment Reporting Information
Revenues	$ 335		$ 331