Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2024	Sep. 30, 2023
Assets [Abstract]
Utility Plant		$ 8,779.1	$ 8,210.1
Less: Accumulated depreciation and amortization		2,535.8	2,431.2
Net Utility Plant		6,243.3	5,778.9
Non-utility Property (net of accumulated depreciation and amortization of $96.8 and $71.1 at September 30, 2024 and 2023, respectively)		955.3	628.5
Other Investments		115.3	102.6
Total Other Property and Investments		1,070.6	731.1
Other Property and Investments		1,070.6	731.1
Assets, Current [Abstract]
Cash and cash equivalents		4.5	5.6
Utility		196.3	192.4
Other		112.5	128.6
Allowance for credit losses		(31.4)	(32.5)
Delayed customer billings		12.0	22.0
Accounts receivable:
Utility		196.3	192.4
Other		112.5	128.6
Allowance for credit losses		(31.4)	(32.5)
Delayed customer billings		12.0	22.0
Inventories:
Natural gas		208.6	223.7
Propane gas		8.6	8.6
Materials and supplies		46.7	47.2
Regulatory Asset, Current		115.4	348.3
Prepayments		47.6	48.2
Other Assets, Current		50.5	84.8
Total Current Assets		771.3	1,076.9
Deferred Charges and Other Assets:
Goodwill		1,171.6	1,171.6
Regulatory assets		1,251.8	1,249.2
Other		352.1	305.9
Total Deferred Charges and Other Assets		2,775.5	2,726.7
Total Assets		10,860.7	10,313.6
Capitalization:
Common stock (par value $1.00 per share; 70.0 million shares authorized; 57.7 million shares and 53.2 million shares issued and outstanding at September 30, 2024, and 2023, respectively)		57.7	53.2
Paid-in capital		1,902.2	1,616.5
Retained earnings		1,018.7	958.0
Accumulated other comprehensive income		12.1	47.6
Total Shareholders' Equity		3,232.7	2,917.3
Temporary equity		8.6	16.5
Long-term debt (less current portion)		3,704.4	3,554.0
Total Capitalization		6,945.7	6,487.8
Liabilities, Current [Abstract]
Current portion of long-term debt		42.0	156.6
Notes payable		947.0	955.5
Accounts payable		237.2	253.1
Advance customer billings		48.4	20.9
Wages and compensation accrued		51.5	47.0
Customer deposits		29.9	27.7
Taxes accrued		105.2	104.1
Regulatory liabilities		49.5	7.3
Other Liabilities, Current		193.2	183.2
Total Current Liabilities		1,703.9	1,755.4
Regulatory liabilities		49.5	7.3
Deferred Credits and Other Liabilities:
Deferred income taxes		808.4	743.7
Pension and postretirement benefit costs		146.7	137.3
Asset retirement obligations		579.9	577.4
Regulatory liabilities		535.5	472.4
Other Liabilities, Noncurrent		140.6	139.6
Total Deferred Credits and Other Liabilities		2,211.1	2,070.4
Commitments and Contingencies (Note 16)
Total Capitalization and Liabilities		10,860.7	10,313.6
Regulatory liabilities		535.5	472.4
Depositary Shares [Member]
Capitalization:
Preferred stock ($25.00 par value per share; 10.0 million depositary shares authorized, issued and outstanding at September 30, 2024 and 2023)		242.0	242.0
Spire Missouri [Member]
Assets [Abstract]
Utility Plant		5,420.2	4,964.9
Less: Accumulated depreciation and amortization		1,086.0	1,043.2
Net Utility Plant		4,334.2	3,921.7
Total Other Property and Investments		70.1	65.7
Other Property and Investments		70.1	65.7
Assets, Current [Abstract]
Cash and cash equivalents		0.0	0.8
Other		22.2	21.1
Allowance for credit losses		(24.9)	(26.2)
Delayed customer billings		5.7	17.9
Accounts receivable:
Other		22.2	21.1
Allowance for credit losses		(24.9)	(26.2)
Delayed customer billings		5.7	17.9
Inventories:
Natural gas		129.6	132.8
Propane gas		8.6	8.6
Materials and supplies		24.4	24.2
Regulatory Asset, Current		84.0	293.1
Prepayments		27.2	26.7
Total Current Assets		432.5	642.9
Deferred Charges and Other Assets:
Goodwill		210.2	210.2
Regulatory assets		588.0	617.6
Other		193.6	147.4
Total Deferred Charges and Other Assets		991.8	975.2
Total Assets		5,828.6	5,605.5
Capitalization:
Common stock (par value $1.00 per share; 70.0 million shares authorized; 57.7 million shares and 53.2 million shares issued and outstanding at September 30, 2024, and 2023, respectively)		0.1	0.1
Paid-in capital		854.8	854.8
Retained earnings		1,110.8	992.4
Accumulated other comprehensive income		(2.0)	(2.5)
Total Shareholders' Equity	[1]	1,963.7	1,844.8
Long-term debt (less current portion)		1,803.4	1,785.4
Total Capitalization		3,767.1	3,630.2
Liabilities, Current [Abstract]
Advance customer billings		35.5	11.0
Wages and compensation accrued		24.2	23.6
Customer deposits		6.1	5.8
Taxes accrued		60.2	60.3
Regulatory liabilities		10.2	0.0
Other Liabilities, Current		50.6	48.7
Total Current Liabilities		781.7	786.3
Regulatory liabilities		10.2	0.0
Deferred Credits and Other Liabilities:
Deferred income taxes		567.6	531.8
Pension and postretirement benefit costs		110.0	103.3
Asset retirement obligations		95.7	111.1
Regulatory liabilities		443.3	389.4
Other Liabilities, Noncurrent		63.2	53.4
Total Deferred Credits and Other Liabilities		1,279.8	1,189.0
Total Capitalization and Liabilities		5,828.6	5,605.5
Regulatory liabilities		443.3	389.4
Spire Missouri [Member] | Nonrelated Party [Member]
Assets, Current [Abstract]
Utility		152.9	142.6
Accounts receivable:
Utility		152.9	142.6
Liabilities, Current [Abstract]
Accounts payable		92.0	85.8
Spire Missouri [Member] | Related Party [Member]
Assets, Current [Abstract]
Utility		2.8	1.3
Accounts receivable:
Utility		2.8	1.3
Liabilities, Current [Abstract]
Notes payable		495.3	540.6
Accounts payable		7.6	10.5
Spire Alabama Inc [Member]
Assets [Abstract]
Utility Plant		2,966.6	2,862.6
Less: Accumulated depreciation and amortization		1,336.6	1,273.0
Net Utility Plant		1,630.0	1,589.6
Total Other Property and Investments		0.1	0.0
Other Property and Investments		0.1	0.0
Assets, Current [Abstract]
Cash and cash equivalents		1.5	1.2
Other		6.1	6.6
Allowance for credit losses		(5.7)	(5.7)
Delayed customer billings		5.7	3.6
Accounts receivable:
Other		6.1	6.6
Allowance for credit losses		(5.7)	(5.7)
Delayed customer billings		5.7	3.6
Inventories:
Natural gas		37.3	52.4
Materials and supplies		18.7	19.1
Regulatory Asset, Current		19.2	41.6
Prepayments		10.7	6.4
Total Current Assets		129.4	168.6
Deferred Charges and Other Assets:
Regulatory assets		642.0	606.9
Other		94.8	84.2
Total Deferred Charges and Other Assets		736.8	691.1
Total Assets		2,496.3	2,449.3
Capitalization:
Retained earnings		668.9	642.1
Total Shareholders' Equity		948.3	928.0
Long-term debt (less current portion)		711.3	745.9
Total Capitalization		1,659.6	1,673.9
Common stock and paid-in capital (par value $0.01 per share; 3,000,000 shares authorized; 1,972,052 issued and outstanding at September 30, 2024 and 2023)		279.4	285.9
Liabilities, Current [Abstract]
Current portion of long-term debt		35.0	0.0
Advance customer billings		10.9	8.1
Wages and compensation accrued		7.2	6.6
Customer deposits		20.8	19.3
Taxes accrued		34.8	34.5
Regulatory liabilities		33.8	0.0
Other Liabilities, Current		13.8	14.7
Total Current Liabilities		249.4	240.3
Regulatory liabilities		33.8	0.0
Deferred Credits and Other Liabilities:
Deferred income taxes		35.9	9.1
Pension and postretirement benefit costs		28.0	27.2
Asset retirement obligations		468.6	451.0
Regulatory liabilities		28.3	21.2
Other Liabilities, Noncurrent		26.5	26.6
Total Deferred Credits and Other Liabilities		587.3	535.1
Total Capitalization and Liabilities		2,496.3	2,449.3
Regulatory liabilities		28.3	21.2
Spire Alabama Inc [Member] | Nonrelated Party [Member]
Assets, Current [Abstract]
Utility		35.5	42.2
Accounts receivable:
Utility		35.5	42.2
Liabilities, Current [Abstract]
Accounts payable		38.0	28.4
Spire Alabama Inc [Member] | Related Party [Member]
Assets, Current [Abstract]
Utility		0.4	1.2
Accounts receivable:
Utility		0.4	1.2
Liabilities, Current [Abstract]
Notes payable		48.4	124.1
Accounts payable		$ 6.7	$ 4.6

[1] Accumulated other comprehensive income (loss)