Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 492,995	$ 485,738	$ 2,346,223	$ 2,458,165
Regulatory mechanism threshold (in percent)			75.00%
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	434,650	444,287	$ 2,194,786	2,339,660
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	58,345	41,451	151,437	118,505
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	437,399	443,705	2,175,036	2,353,535
Alternative revenue program revenues	(2,567)	748	20,320	(13,388)
Other Revenues	476	491	1,461	1,521
Total operating revenues	435,308	444,944	2,196,817	2,341,668
Operating Segments | Distribution | Gas sales revenues:
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	412,074	414,657	2,080,795	2,257,091
Operating Segments | Distribution | Gas sales revenues: | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	286,937	269,484	1,435,328	1,513,239
Operating Segments | Distribution | Gas sales revenues: | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	101,055	113,591	543,148	611,474
Operating Segments | Distribution | Gas sales revenues: | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,019	25,277	67,572	95,701
Operating Segments | Distribution | Gas sales revenues: | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,063	6,305	34,747	36,677
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	22,532	22,923	77,676	76,005
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,793	6,125	16,565	20,439
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	166,952	169,271	480,200	463,420
Alternative revenue program revenues	(8,944)	(20,073)	(27,779)	(44,102)
Other Revenues	0	0	0	0
Total operating revenues	158,008	149,198	452,421	419,318
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	164,675	166,864	471,433	456,558
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 2,277	$ 2,407	$ 8,767	$ 6,862