Segment Information - Schedule of Information about Segment Revenue, Segment Expenses and Adjusted Earnings (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Revenues from external customers		$ 2,476.4	$ 2,593.0	$ 2,666.3
Intersegment revenues		0.0	0.0	0.0
Total Operating Revenues		2,476.4	2,593.0	2,666.3
Cost of gas sold, incl. gross receipts taxes		1,021.2	1,231.5	1,392.6
Operation and maintenance expense		542.1	507.4	517.6
Depreciation and amortization expense		298.2	278.4	254.8
Interest Expense		204.1	201.1	185.7
Income tax expense (benefit)		59.7	58.7	38.8
Other segment items	[1]	75.6	68.5	48.7
Adjusted Earnings (Loss)		275.5	247.4	228.1
Capital expenditures		922.4	861.3	662.5
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenues from external customers		2,473.9	2,592.3	2,665.6
Intersegment revenues		46.4	45.5	36.5
Total Operating Revenues		2,520.3	2,637.8	2,702.1
Cost of gas sold, incl. gross receipts taxes		1,067.6	1,276.9	1,429.1
Operation and maintenance expense		531.8	505.7	511.7
Depreciation and amortization expense		297.8	277.9	254.3
Interest Expense		148.7	154.3	148.5
Income tax expense (benefit)		74.2	68.4	49.3
Other segment items	[1]	86.6	76.9	47.0
Adjusted Earnings (Loss)		313.6	277.7	262.2
Capital expenditures		923.7	862.5	662.6
Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenues from external customers		0.0	0.0	0.0
Intersegment revenues		(46.4)	(45.5)	(36.5)
Total Operating Revenues		(46.4)	(45.5)	(36.5)
Cost of gas sold, incl. gross receipts taxes		(46.4)	(45.5)	(36.5)
Operation and maintenance expense		0.0	0.0	0.0
Depreciation and amortization expense		0.0	0.0	0.0
Interest Expense		0.0	0.0	0.0
Income tax expense (benefit)		0.0	0.0	0.0
Other segment items	[1]	0.0	0.0	0.0
Adjusted Earnings (Loss)		0.0	0.0	0.0
Capital expenditures		0.0	0.0	0.0
Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenues from external customers		2,207.5	2,436.2	2,456.6
Intersegment revenues		0.1	1.7	0.3
Total Operating Revenues		2,207.6	2,437.9	2,456.9
Cost of gas sold, incl. gross receipts taxes		970.9	1,238.7	1,321.1
Operation and maintenance expense		467.1	452.8	461.8
Depreciation and amortization expense		277.6	263.6	244.4
Interest Expense		137.1	147.3	139.9
Income tax expense (benefit)		47.4	48.0	32.7
Other segment items	[1]	76.1	66.7	56.5
Adjusted Earnings (Loss)		231.4	220.8	200.5
Capital expenditures		816.8	691.1	588.6
Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenues from external customers		157.2	99.2	179.1
Intersegment revenues		0.0	0.0	0.0
Total Operating Revenues		157.2	99.2	179.1
Cost of gas sold, incl. gross receipts taxes		93.7	37.1	108.0
Operation and maintenance expense		19.4	18.2	19.4
Depreciation and amortization expense		1.0	1.5	1.5
Interest Expense		0.0	0.0	0.0
Income tax expense (benefit)		11.1	10.8	12.8
Other segment items	[1]	6.1	8.2	(10.2)
Adjusted Earnings (Loss)		25.9	23.4	47.6
Capital expenditures		0.1	0.1	0.4
Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenues from external customers		109.2	56.9	29.9
Intersegment revenues		46.3	43.8	36.2
Total Operating Revenues		155.5	100.7	66.1
Cost of gas sold, incl. gross receipts taxes		3.0	1.1	0.0
Operation and maintenance expense		45.3	34.7	30.5
Depreciation and amortization expense		19.2	12.8	8.4
Interest Expense		11.6	7.0	8.6
Income tax expense (benefit)		15.7	9.6	3.8
Other segment items	[1]	4.4	2.0	0.7
Adjusted Earnings (Loss)		56.3	33.5	14.1
Capital expenditures		106.8	171.3	73.6
Other [Member]
Disaggregation of Revenue [Line Items]
Revenues from external customers	[2]	2.5	0.7	0.7
Intersegment revenues	[2]	0.0	0.0	0.0
Total Operating Revenues	[2]	2.5	0.7	0.7
Cost of gas sold, incl. gross receipts taxes	[2]	0.0	0.1	0.0
Operation and maintenance expense	[2]	10.3	1.7	5.9
Depreciation and amortization expense	[2]	0.4	0.5	0.5
Interest Expense	[2]	55.4	46.8	37.2
Income tax expense (benefit)	[2]	(14.5)	(9.7)	(10.5)
Other segment items	[1],[2]	(11.0)	(8.4)	1.7
Adjusted Earnings (Loss)	[2]	(38.1)	(30.3)	(34.1)
Capital expenditures	[2]	$ (1.3)	$ (1.2)	$ (0.1)

[1] Other segment items for each reportable segment include fair value and timing adjustments, acquisition and restructuring activities, taxes other than income and gross receipt taxes, and miscellaneous income and deductions.

[2] All other components of the Company's consolidated information include Spire's subsidiaries engaged in the operation of a propane pipeline and risk management, among other activities, and unallocated corporate items, including certain debt and associated interest costs.