Revenue and Accounts Receivable - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,950,502	$ 1,647,227	$ 3,126,501	$ 2,805,694
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	1,881,742	1,588,394	2,990,311	2,693,013
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	68,760	58,833	136,190	112,681
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,899,179	1,562,867	2,937,234	2,647,027
Alternative revenue program revenues	(20,117)	22,315	47,219	39,716
Other revenues	3,466	3,999	7,410	7,776
Total operating revenues	1,882,528	1,589,181	2,991,863	2,694,519
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	270,773	225,321	539,466	443,668
Alternative revenue program revenues	(11,774)	(1,834)	(25,077)	(9,012)
Other revenues	0	0	0	0
Total operating revenues	258,999	223,487	514,389	434,656
Operating Segments | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,851,605	1,521,536	2,849,911	2,569,286
Operating Segments | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Transportation revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	43,352	37,607	80,079	71,374
Operating Segments | Transportation revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	266,514	223,159	532,543	438,464
Operating Segments | Miscellaneous revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,222	3,724	7,244	6,367
Operating Segments | Miscellaneous revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,259	2,162	6,923	5,204
Operating Segments | Residential | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,308,691	1,065,296	2,001,741	1,792,978
Operating Segments | Residential | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Commercial | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	480,108	404,701	746,162	681,954
Operating Segments | Commercial | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Industrial | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	38,885	30,419	65,206	58,650
Operating Segments | Industrial | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Public authority and other | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	23,921	21,120	36,802	35,704
Operating Segments | Public authority and other | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0	$ 0	$ 0