Business Segment Information: Information Relating to Segment Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information
Revenue from contracts with customers	$ 597,970	$ 573,355
Revenues	597,810	575,389
Operating income	160,131	148,274
Interest expense, net	(34,717)	(34,995)
Other income (expense), net	(789)	(104)
Income tax benefit (expense)	(17,263)	25,802
Income from continuing operations	107,362	138,977
Net (loss) from discontinued operations	0	(2,343)
Net income	107,362	136,634
Net income attributable to noncontrolling interest	(3,554)	(3,630)
Net income available for common stock	103,808	133,004
Changes To Our Segment Performance Measure - Revenues	0
Changes To Our Segment Performance Measure - Operating Income	0
Deferred income taxes	19,602	(25,430)
Other Restructuring
Segment Reporting Information
Deferred income taxes		49,000
Recasting of Intercompany Capital Lease
Segment Reporting Information
Changes To Our Segment Performance Measure - Revenues		0
Changes To Our Segment Performance Measure - Operating Income		0
Inter-company Revenues
Segment Reporting Information
Revenue from contracts with customers	(28,200)	(27,453)
Revenues	(37,871)	(36,577)
Consolidation, Eliminations
Segment Reporting Information
Revenue from contracts with customers	0	0
Corporate
Segment Reporting Information
Operating income	(507)	(1,696)
Changes To Our Segment Performance Measure - Operating Income	4,700
Corporate \| Recasting of Intercompany Capital Lease
Segment Reporting Information
Changes To Our Segment Performance Measure - Operating Income		(100)
Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	182,673	173,322
Revenues	182,927	173,555
Operating income	41,020	38,480
Electric Utilities \| Recasting of Intercompany Capital Lease
Segment Reporting Information
Changes To Our Segment Performance Measure - Operating Income		1,700
Electric Utilities \| Segment Reconciling Items [Member]
Segment Reporting Information
Changes To Our Segment Performance Measure - Operating Income	(5,400)
Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	412,199	396,160
Revenues	411,080	397,344
Operating income	103,314	95,443
Power Generation
Segment Reporting Information
Revenue from contracts with customers	15,469	14,769
Revenues	25,245	23,939
Operating income	11,967	11,776
Changes To Our Segment Performance Measure - Revenues	3,400
Power Generation \| Recasting of Intercompany Capital Lease
Segment Reporting Information
Changes To Our Segment Performance Measure - Revenues		800
Changes To Our Segment Performance Measure - Operating Income		(1,600)
Power Generation \| Segment Reconciling Items [Member]
Segment Reporting Information
Changes To Our Segment Performance Measure - Operating Income	700
Mining
Segment Reporting Information
Revenue from contracts with customers	15,829	16,557
Revenues	16,429	17,128
Operating income	4,337	4,271
Other Revenues
Segment Reporting Information
Revenues	(160)	2,034
Other Revenues \| Inter-company Revenues
Segment Reporting Information
Revenues	(9,671)	(9,124)
Other Revenues \| Consolidation, Eliminations
Segment Reporting Information
Revenues	0	0
Other Revenues \| Electric Utilities
Segment Reporting Information
Revenues	254	233
Other Revenues \| Gas Utilities
Segment Reporting Information
Revenues	(1,119)	1,184
Other Revenues \| Power Generation
Segment Reporting Information
Revenues	9,776	9,170
Other Revenues \| Mining
Segment Reporting Information
Revenues	600	571
External Operating Revenue
Segment Reporting Information
Revenue from contracts with customers	597,970	573,355
External Operating Revenue \| Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	176,663	167,178
External Operating Revenue \| Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	411,500	395,742
External Operating Revenue \| Power Generation
Segment Reporting Information
Revenue from contracts with customers	2,257	1,720
External Operating Revenue \| Mining
Segment Reporting Information
Revenue from contracts with customers	7,550	8,715
External Operating Revenue \| Other Revenues
Segment Reporting Information
Revenues	(160)	2,034
External Operating Revenue \| Other Revenues \| Electric Utilities
Segment Reporting Information
Revenues	254	233
External Operating Revenue \| Other Revenues \| Gas Utilities
Segment Reporting Information
Revenues	(1,119)	1,184
External Operating Revenue \| Other Revenues \| Power Generation
Segment Reporting Information
Revenues	436	371
External Operating Revenue \| Other Revenues \| Mining
Segment Reporting Information
Revenues	269	246
Inter-company Operating Revenue \| Electric Utilities
Segment Reporting Information
Revenue from contracts with customers	6,010	6,144
Inter-company Operating Revenue \| Gas Utilities
Segment Reporting Information
Revenue from contracts with customers	699	418
Inter-company Operating Revenue \| Power Generation
Segment Reporting Information
Revenue from contracts with customers	13,212	13,049
Inter-company Operating Revenue \| Mining
Segment Reporting Information
Revenue from contracts with customers	8,279	7,842
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	9,340	8,799
Inter-company Operating Revenue \| Other Revenues \| Mining
Segment Reporting Information
Revenues	$ 331	$ 325

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