Revenue and Accounts Receivable - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 605,553	$ 492,995	$ 2,839,106	$ 2,346,223
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	557,931	434,650	$ 2,715,644	2,194,786
Regulatory mechanism threshold (in percent)			75.00%
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	47,622	58,345	$ 123,462	151,437
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	556,066	437,399	2,730,273	2,175,036
Alternative revenue program revenues	2,206	(2,567)	(13,666)	20,320
Other revenues	478	476	1,467	1,461
Total operating revenues	558,750	435,308	2,718,074	2,196,817
Operating Segments | Distribution | Gas sales revenues:
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	527,548	412,074	2,637,294	2,080,795
Operating Segments | Distribution | Gas sales revenues: | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	336,016	286,937	1,821,570	1,435,328
Operating Segments | Distribution | Gas sales revenues: | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	157,314	101,055	692,443	543,148
Operating Segments | Distribution | Gas sales revenues: | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	25,348	17,019	81,122	67,572
Operating Segments | Distribution | Gas sales revenues: | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,870	7,063	42,159	34,747
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	25,903	22,532	84,643	77,676
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,615	2,793	8,336	16,565
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	168,514	166,952	493,866	480,200
Alternative revenue program revenues	(5,527)	(8,944)	(16,998)	(27,779)
Other revenues	0	0	0	0
Total operating revenues	162,987	158,008	476,868	452,421
Operating Segments | Pipeline and Storage | Gas sales revenues:
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues: | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues: | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues: | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues: | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Pipeline and Storage | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	164,619	164,675	480,945	471,433
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 3,895	$ 2,277	$ 12,921	$ 8,767