CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating revenues
Total operating revenues	$ 2,821,137	$ 2,901,848	$ 3,115,546
Total purchased gas cost	658,854	858,837	1,167,848
Operation and maintenance expense	629,601	630,308	594,795
Depreciation and amortization expense	429,828	391,456	361,083
Taxes, other than income	278,755	275,189	263,886
Operating income	824,099	746,058	727,934
Other non-operating income (expense)	7,171	7,404	(10,144)
Interest charges	84,474	103,153	106,646
Income from continuing operations before income taxes	746,796	650,309	611,144
Income tax expense	145,353	138,903	8,080
Net Income	$ 601,443	$ 511,406	$ 603,064
Basic net income per share	$ 4.89	$ 4.36	$ 5.43
Diluted net income per share	$ 4.89	$ 4.35	$ 5.43
Weighted average shares outstanding - basic	122,788	117,200	111,012
Weighted average shares outstanding - diluted	122,872	117,461	111,012
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net unrealized holding gains (losses) on available-for-sale securities, net of tax	$ 106	$ 218	$ (395)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax, Parent	56,888	(22,944)	44,936
Total other comprehensive income (loss)	56,994	(22,726)	44,541
Total comprehensive income	658,437	488,680	647,605
Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	658,854	858,837	1,167,848
Distribution
Operating revenues
Total operating revenues	2,624,251	2,742,824	3,000,404
Pipeline and Storage
Operating revenues
Total operating revenues	196,886	159,024	115,142
Operating Segments | Distribution
Operating revenues
Total operating revenues	2,626,993	2,745,461	3,003,047
Total purchased gas cost	1,071,227	1,268,591	1,559,836
Operation and maintenance expense	472,760	480,222	461,048
Depreciation and amortization expense	309,582	283,697	264,930
Taxes, other than income	245,181	242,179	231,566
Operating income	528,243	470,772	485,667
Other non-operating income (expense)	(1,265)	6,241	(6,649)
Interest charges	39,634	60,031	65,850
Income from continuing operations before income taxes	487,344	416,982	413,168
Income tax expense	91,680	88,168	(29,798)
Net Income	395,664	328,814	442,966
Operating Segments | Distribution | Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	1,071,227	1,268,591	1,559,836
Operating Segments | Pipeline and Storage
Operating revenues
Total operating revenues	609,339	567,024	507,713
Total purchased gas cost	1,548	(360)	1,978
Operation and maintenance expense	158,115	151,329	134,995
Depreciation and amortization expense	120,246	107,759	96,153
Taxes, other than income	33,574	33,010	32,320
Operating income	295,856	275,286	242,267
Other non-operating income (expense)	8,436	1,163	(3,495)
Interest charges	44,840	43,122	40,796
Income from continuing operations before income taxes	259,452	233,327	197,976
Income tax expense	53,673	50,735	37,878
Net Income	205,779	182,592	160,098
Operating Segments | Pipeline and Storage | Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	1,548	(360)	1,978
Eliminations
Operating revenues
Total operating revenues	(415,195)	(410,637)	(395,214)
Total purchased gas cost	(413,921)	(409,394)	(393,966)
Operation and maintenance expense	(1,274)	(1,243)	(1,248)
Depreciation and amortization expense	0	0	0
Taxes, other than income	0	0	0
Operating income	0	0	0
Other non-operating income (expense)	0	0	0
Interest charges	0	0	0
Income from continuing operations before income taxes	0	0	0
Income tax expense	0	0	0
Net Income	0	0	0
Eliminations | Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	(413,921)	(409,394)	(393,966)
Eliminations | Distribution
Operating revenues
Total operating revenues	2,742	2,637	2,643
Eliminations | Pipeline and Storage
Operating revenues
Total operating revenues	$ 412,453	$ 408,000	$ 392,571