CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Operating revenues
Total operating revenues	$ 2,901,848	$ 3,115,546	$ 2,759,735
Operation and maintenance expense	630,308	594,795	538,716
Depreciation and amortization expense	391,456	361,083	319,448
Taxes, other than income	275,189	263,886	240,407
Operating income	746,058	727,934	735,628
Other non-operating income (expense)	7,404	(10,144)	(11,352)
Interest charges	103,153	106,646	120,182
Income from continuing operations before income taxes	650,309	611,144	604,094
Income tax expense	138,903	8,080	221,383
Income from continuing operations	511,406	603,064	382,711
Income from discontinued operations, net of tax	0	0	10,994
Gain on sale of discontinued operations, net of tax	0	0	2,716
Net Income	$ 511,406	$ 603,064	$ 396,421
Earnings Per Share, Basic and Diluted [Abstract]
Income per share from continuing operations - basic	$ 4.36	$ 5.43	$ 3.60
Income per share from discontinued operations - basic	0	0	0.13
Net income per share - basic	4.36	5.43	3.73
Income per share from continuing operations - diluted	4.35	5.43	3.60
Income per share from discontinued operations - diluted	0	0	0.13
Net income per share - diluted	$ 4.35	$ 5.43	$ 3.73
Weighted average shares outstanding - basic	117,200	111,012	106,100
Weighted average shares outstanding - diluted	117,461	111,012	106,100
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net unrealized holding gains (losses) on available-for-sale securities, net of tax	$ 218	$ (395)	$ 2,564
Cash Flow Hedges [Abstract]
Amortization and unrealized gain on interest rate agreements, net of tax	(22,944)	44,936	75,222
Net unrealized gains on commodity cash flow hedges, net of tax	0	0	4,982
Total other comprehensive income (loss)	(22,726)	44,541	82,768
Total comprehensive income	488,680	647,605	479,189
Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	858,837	1,167,848	925,536
Operating Segments | Distribution
Operating revenues
Total operating revenues	2,745,461	3,003,047	2,649,175
Operation and maintenance expense	480,222	461,048	404,995
Depreciation and amortization expense	283,697	264,930	249,071
Taxes, other than income	242,179	231,566	211,929
Operating income	470,772	485,667	513,724
Other non-operating income (expense)	6,241	(6,649)	(9,777)
Interest charges	60,031	65,850	79,789
Income from continuing operations before income taxes	416,982	413,168	424,158
Income tax expense	88,168	(29,798)	155,789
Income from continuing operations			268,369
Income from discontinued operations, net of tax			0
Gain on sale of discontinued operations, net of tax			0
Net Income	328,814	442,966	268,369
Operating Segments | Distribution | Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	1,268,591	1,559,836	1,269,456
Operating Segments | Pipeline and Storage
Operating revenues
Total operating revenues	567,024	507,713	457,030
Operation and maintenance expense	151,329	134,995	133,765
Depreciation and amortization expense	107,759	96,153	70,377
Taxes, other than income	33,010	32,320	28,478
Operating income	275,286	242,267	221,904
Other non-operating income (expense)	1,163	(3,495)	(1,575)
Interest charges	43,122	40,796	40,393
Income from continuing operations before income taxes	233,327	197,976	179,936
Income tax expense	50,735	37,878	65,594
Income from continuing operations			114,342
Income from discontinued operations, net of tax			0
Gain on sale of discontinued operations, net of tax			0
Net Income	182,592	160,098	114,342
Operating Segments | Pipeline and Storage | Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	(360)	1,978	2,506
Eliminations
Operating revenues
Total operating revenues	(410,637)	(395,214)	(346,470)
Operation and maintenance expense	(1,243)	(1,248)	(44)
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
Income from continuing operations			0
Income from discontinued operations, net of tax			0
Gain on sale of discontinued operations, net of tax			0
Net Income	0	0	0
Eliminations | Natural Gas, US Regulated
Operating revenues
Total purchased gas cost	$ (409,394)	$ (393,966)	$ (346,426)