Business Segment Information (Segment Information By Segment) (Details) - USD ($) $ in Thousands		3 Months Ended												12 Months Ended
		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenue		$ 293,341	$ 357,200	$ 552,544	$ 490,247		$ 289,196		$ 342,912	$ 540,905		$ 419,655		$ 1,693,332	$ 1,592,668	$ 1,579,881
Interest Income														6,065	6,766	4,113
Interest Expense														106,756	114,522	119,837
Depreciation, Depletion and Amortization														275,660	240,961	224,195
Income Tax Expense (Benefit)														85,221	(7,494)	160,682
Segment Profit: Net Income (Loss)		47,282	$ 63,753	$ 90,595	$ 102,660	[1]	37,995	[2]	$ 63,025	$ 91,847	[3]	$ 198,654	[4]	304,290	391,521	283,482
Expenditures for Additions to Long-Lived Assets														781,246	600,602	462,117
Segment Assets		6,462,157					6,036,486							6,462,157	6,036,486	6,103,320
Utility [Member]
Segment Reporting Information [Line Items]
Revenue														727,442
Interest Income														1,809	1,591	1,051
Interest Expense														23,443	26,753	28,492
Depreciation, Depletion and Amortization														53,832	53,253	52,582
Income Tax Expense (Benefit)														13,967	15,258	24,894
Segment Profit: Net Income (Loss)														60,871	51,217	46,935
Expenditures for Additions to Long-Lived Assets														95,847	85,648	80,867
Segment Assets		1,991,338					1,921,971							1,991,338	1,921,971	2,013,123
Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Revenue														288,283
Interest Income														2,982	2,748	1,467
Interest Expense														29,142	31,383	33,717
Depreciation, Depletion and Amortization														44,947	43,463	41,196
Income Tax Expense (Benefit)														23,238	17,806	40,947
Segment Profit: Net Income (Loss)														74,011	97,246	68,446
Expenditures for Additions to Long-Lived Assets														143,005	92,832	95,336
Segment Assets		1,893,514					1,848,180							1,893,514	1,848,180	1,929,788
Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue														632,740
Interest Income														1,107	1,479	707
Interest Expense														54,777	54,288	53,702
Depreciation, Depletion and Amortization														154,784	124,274	112,565
Income Tax Expense (Benefit)														32,978	(41,962)	66,093
Segment Profit: Net Income (Loss)														111,807	180,632	129,326
Expenditures for Additions to Long-Lived Assets														491,889	380,677	253,057
Segment Assets		1,972,776					1,568,563							1,972,776	1,568,563	1,407,152
Gathering [Member]
Segment Reporting Information [Line Items]
Revenue														127,075
Interest Income														546	1,106	994
Interest Expense														9,406	9,560	9,142
Depreciation, Depletion and Amortization														20,038	17,313	16,162
Income Tax Expense (Benefit)														20,895	(17,677)	29,694
Segment Profit: Net Income (Loss)														58,413	83,519	40,377
Expenditures for Additions to Long-Lived Assets														49,650	61,728	32,645
Segment Assets		547,995					533,608							547,995	533,608	580,051
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue														1,775,540
Interest Income														6,444	6,924	4,219
Interest Expense														116,768	121,984	125,053
Depreciation, Depletion and Amortization														273,601	238,303	222,505
Income Tax Expense (Benefit)														91,078	(26,575)	161,628
Segment Profit: Net Income (Loss)														305,102	412,614	285,084
Expenditures for Additions to Long-Lived Assets														780,391	620,885	461,905
Segment Assets		6,405,623					5,872,322							6,405,623	5,872,322	5,930,114
All Other [Member]
Segment Reporting Information [Line Items]
Revenue														149,709
Interest Income														1,291	1,073	784
Interest Expense														21	22	47
Depreciation, Depletion and Amortization														1,291	1,902	940
Income Tax Expense (Benefit)														(955)	2,125	644
Segment Profit: Net Income (Loss)														(1,811)	261	1,167
Expenditures for Additions to Long-Lived Assets														128	41	75
Segment Assets		122,241					129,080							122,241	129,080	137,798
Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue														(231,917)
Interest Income														(1,670)	(1,231)	(890)
Interest Expense														(10,033)	(7,484)	(5,263)
Depreciation, Depletion and Amortization														768	756	750
Income Tax Expense (Benefit)														(4,902)	16,956	(1,590)
Segment Profit: Net Income (Loss)														999	(21,354)	(2,769)
Expenditures for Additions to Long-Lived Assets														727	(20,324)	137
Segment Assets		$ (65,707)					$ 35,084							(65,707)	35,084	35,408
Revenue from External Customers [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													1,693,332	1,592,668	1,579,881
Revenue from External Customers [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													715,813	674,726	626,899
Revenue from External Customers [Member] | Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													195,808	210,345	206,615
Revenue from External Customers [Member] | Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													632,740	564,547	614,599
Revenue from External Customers [Member] | Gathering [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													11	41	115
Revenue from External Customers [Member] | Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													1,544,372	1,449,659	1,448,228
Revenue from External Customers [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													148,582	142,349	130,759
Revenue from External Customers [Member] | Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	[5]													378	660	894
Intersegment Revenues [Member]
Segment Reporting Information [Line Items]
Revenue														0	0	0
Intersegment Revenues [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenue														11,629	12,800	13,072
Intersegment Revenues [Member] | Pipeline And Storage [Member]
Segment Reporting Information [Line Items]
Revenue														92,475	89,981	87,810
Intersegment Revenues [Member] | Exploration And Production [Member]
Segment Reporting Information [Line Items]
Revenue														0	0	0
Intersegment Revenues [Member] | Gathering [Member]
Segment Reporting Information [Line Items]
Revenue														127,064	107,856	107,566
Intersegment Revenues [Member] | Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue														231,168	210,637	208,448
Intersegment Revenues [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Revenue														1,127	826	794
Intersegment Revenues [Member] | Corporate And Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue														$ (232,295)	$ (211,463)	$ (209,242)

[1] Includes a $5.0 million reduction to income tax expense associated with the remeasurement of accumulated deferred income taxes in accordance with the 2017 Tax Reform Act.

[2] Includes a $3.5 million increase to income tax expense associated with the remeasurement of accumulated deferred income taxes in accordance with the 2017 Tax Reform Act.

[3] Includes a $4.0 million increase to income tax expense associated with the remeasurement of accumulated deferred income taxes in accordance with the 2017 Tax Reform Act.

[4] Includes a $111.0 million reduction to income tax expense associated with the remeasurement of accumulated deferred income taxes in accordance with the 2017 Tax Reform Act.

[5] All Revenue from External Customers originated in the United States.