Segment Information - Schedule of Information about Segment Revenue, Segment Expenses and Adjusted Earnings (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues from external customers		$ 762.2	$ 669.1
Intersegment revenues		0.0	0.0
Total Operating Revenues		762.2	669.1
Cost of gas sold, incl. gross receipts taxes		342.6	296.8
Operation and maintenance expense		139.9	129.3
Depreciation and amortization expense		81.4	72.3
Interest expense		60.4	48.0
Income tax expense (benefit)		23.4	20.1
Other segment items	[1]	6.1	21.5
Adjusted earnings (loss)		108.4	81.1
Capital expenditures		202.8	260.6
Other [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	[2]	1.0	0.4
Intersegment revenues	[2]	0.0	0.0
Total Operating Revenues	[2]	1.0	0.4
Cost of gas sold, incl. gross receipts taxes	[2]	0.0	0.0
Operation and maintenance expense	[2]	6.4	(0.7)
Depreciation and amortization expense	[2]	0.2	0.1
Interest expense	[2]	18.7	11.7
Income tax expense (benefit)	[2]	(3.8)	(0.6)
Other segment items	[1],[2]	(7.8)	0.8
Adjusted earnings (loss)	[2]	(12.7)	(10.9)
Capital expenditures	[2]	(0.4)	(0.1)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		761.2	668.7
Intersegment revenues		12.2	11.3
Total Operating Revenues		773.4	680.0
Cost of gas sold, incl. gross receipts taxes		354.8	308.1
Operation and maintenance expense		133.5	130.0
Depreciation and amortization expense		81.2	72.2
Interest expense		41.7	36.3
Income tax expense (benefit)		27.2	20.7
Other segment items	[1]	13.9	20.7
Adjusted earnings (loss)		121.1	92.0
Capital expenditures		203.2	260.7
Operating Segments [Member] | Gas Utility [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		693.3	613.5
Intersegment revenues		0.0	0.0
Total Operating Revenues		693.3	613.5
Cost of gas sold, incl. gross receipts taxes		313.1	281.3
Operation and maintenance expense		119.7	115.0
Depreciation and amortization expense		74.8	68.1
Interest expense		36.8	35.1
Income tax expense (benefit)		24.1	16.1
Other segment items	[1]	20.9	20.1
Adjusted earnings (loss)		103.9	77.8
Capital expenditures		197.1	217.9
Operating Segments [Member] | Gas Marketing [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		41.1	33.0
Intersegment revenues		0.0	0.0
Total Operating Revenues		41.1	33.0
Cost of gas sold, incl. gross receipts taxes		40.9	26.1
Operation and maintenance expense		4.0	4.0
Depreciation and amortization expense		0.1	0.4
Interest expense		0.0	0.0
Income tax expense (benefit)		(0.8)	0.8
Other segment items	[1]	(7.6)	(0.5)
Adjusted earnings (loss)		4.5	2.2
Capital expenditures		0.0	0.1
Operating Segments [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		26.8	22.2
Intersegment revenues		12.2	11.3
Total Operating Revenues		39.0	33.5
Cost of gas sold, incl. gross receipts taxes		0.8	0.7
Operation and maintenance expense		9.8	11.0
Depreciation and amortization expense		6.3	3.7
Interest expense		4.9	1.2
Income tax expense (benefit)		3.9	3.8
Other segment items	[1]	0.6	1.1
Adjusted earnings (loss)		12.7	12.0
Capital expenditures		6.1	42.7
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		0.0	0.0
Intersegment revenues		(12.2)	(11.3)
Total Operating Revenues		(12.2)	(11.3)
Cost of gas sold, incl. gross receipts taxes		(12.2)	(11.3)
Operation and maintenance expense		0.0	0.0
Depreciation and amortization expense		0.0	0.0
Interest expense		0.0	0.0
Income tax expense (benefit)		0.0	0.0
Other segment items	[1]	0.0	0.0
Adjusted earnings (loss)		0.0	0.0
Capital expenditures		$ 0.0	$ 0.0

[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 risk management, among other activities, and unallocated corporate items, including certain debt and associated interest costs.