Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
ASSETS
Utility Plant	$ 9,333.9	$ 8,779.1
Less: Accumulated depreciation and amortization	2,577.4	2,535.8
Net Utility Plant	6,756.5	6,243.3
Non-utility Property (net of accumulated depreciation and amortization of $129.4 and $96.8 at September 30, 2025 and 2024, respectively)	1,007.2	955.3
Other Investments	128.0	115.3
Total Other Property and Investments	1,135.2	1,070.6
Current Assets:
Cash and cash equivalents	5.7	4.5
Accounts receivable:
Utility	191.9	196.3
Other	152.7	112.5
Allowance for credit losses	(28.8)	(31.4)
Delayed customer billings	13.6	12.0
Inventories:
Natural gas	226.9	208.6
Propane gas	8.6	8.6
Materials and supplies	47.0	46.7
Regulatory Assets, Current	78.3	115.4
Prepayments	47.8	47.6
Other	64.0	50.5
Total Current Assets	807.7	771.3
Deferred Charges and Other Assets:
Goodwill	1,171.6	1,171.6
Regulatory assets	1,323.5	1,251.8
Other	380.8	352.1
Total Deferred Charges and Other Assets	2,875.9	2,775.5
Total Assets	11,575.3	10,860.7
Capitalization:
Common stock	59.0	57.7
Paid-in capital	1,981.4	1,902.2
Retained earnings	1,087.6	1,018.7
Accumulated other comprehensive income	19.4	12.1
Total Shareholders' Equity	3,389.4	3,232.7
Temporary equity	6.1	8.6
Long-term debt (less current portion)	3,369.4	3,704.4
Total Capitalization	6,764.9	6,945.7
Current Liabilities:
Current portion of long-term debt	487.5	42.0
Notes payable	1,317.0	947.0
Accounts payable	248.3	237.2
Advance customer billings	58.1	48.4
Wages and compensation accrued	54.1	51.5
Customer deposits	32.8	29.9
Taxes accrued	109.1	105.2
Regulatory liabilities	39.4	49.5
Other	202.3	193.2
Total Current Liabilities	2,548.6	1,703.9
Deferred Credits and Other Liabilities:
Deferred income taxes	887.4	808.4
Pension and postretirement benefit costs	74.7	146.7
Asset retirement obligations	583.2	579.9
Regulatory liabilities	578.0	535.5
Other	138.5	140.6
Total Deferred Credits and Other Liabilities	2,261.8	2,211.1
Commitments and Contingencies (Note 16)
Total Capitalization and Liabilities	11,575.3	10,860.7
Depositary Shares [Member]
Capitalization:
Preferred stock ($25.00 par value per share; 10.0 million depositary shares authorized, issued and outstanding at September 30, 2025 and 2024)	242.0	242.0
Spire Missouri [Member]
ASSETS
Utility Plant	5,864.3	5,420.2
Less: Accumulated depreciation and amortization	1,104.6	1,086.0
Net Utility Plant	4,759.7	4,334.2
Total Other Property and Investments	75.8	70.1
Accounts receivable:
Other	34.0	22.2
Allowance for credit losses	(23.0)	(24.9)
Delayed customer billings	4.2	5.7
Inventories:
Natural gas	150.0	129.6
Propane gas	8.6	8.6
Materials and supplies	24.5	24.4
Regulatory Assets, Current	48.9	84.0
Prepayments	28.2	27.2
Total Current Assets	426.7	432.5
Deferred Charges and Other Assets:
Goodwill	210.2	210.2
Regulatory assets	653.2	588.0
Other	224.2	193.6
Total Deferred Charges and Other Assets	1,087.6	991.8
Total Assets	6,349.8	5,828.6
Capitalization:
Common stock	0.1	0.1
Paid-in capital	929.2	854.8
Retained earnings	1,239.1	1,110.8
Accumulated other comprehensive income	(0.7)	(2.0)
Total Shareholders' Equity	2,167.7	1,963.7
Long-term debt (less current portion)	1,953.6	1,803.4
Total Capitalization	4,121.3	3,767.1
Current Liabilities:
Advance customer billings	43.2	35.5
Wages and compensation accrued	27.0	24.2
Customer deposits	7.2	6.1
Taxes accrued	65.8	60.2
Regulatory liabilities	13.9	10.2
Other	64.3	50.6
Total Current Liabilities	908.0	781.7
Deferred Credits and Other Liabilities:
Deferred income taxes	598.7	567.6
Pension and postretirement benefit costs	72.4	110.0
Asset retirement obligations	95.5	95.7
Regulatory liabilities	481.3	443.3
Other	72.6	63.2
Total Deferred Credits and Other Liabilities	1,320.5	1,279.8
Commitments and Contingencies (Note 16)
Total Capitalization and Liabilities	6,349.8	5,828.6
Spire Missouri [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	149.1	152.9
Current Liabilities:
Accounts payable	98.9	92.0
Spire Missouri [Member] | Related Party [Member]
Accounts receivable:
Utility	2.2	2.8
Current Liabilities:
Notes payable	566.3	495.3
Accounts payable	21.4	7.6
Spire Alabama Inc [Member]
ASSETS
Utility Plant	3,052.9	2,966.6
Less: Accumulated depreciation and amortization	1,362.4	1,336.6
Net Utility Plant	1,690.5	1,630.0
Total Other Property and Investments	0.1	0.1
Current Assets:
Cash and cash equivalents	1.9	1.5
Accounts receivable:
Other	6.9	6.1
Allowance for credit losses	(4.9)	(5.7)
Delayed customer billings	9.0	5.7
Inventories:
Natural gas	39.4	37.3
Materials and supplies	19.0	18.7
Regulatory Assets, Current	16.1	19.2
Prepayments	7.7	10.7
Total Current Assets	129.9	129.4
Deferred Charges and Other Assets:
Regulatory assets	650.6	642.0
Other	99.2	94.8
Total Deferred Charges and Other Assets	749.8	736.8
Total Assets	2,570.3	2,496.3
Capitalization:
Retained earnings	735.2	668.9
Total Shareholders' Equity	978.1	948.3
Long-term debt (less current portion)	711.7	711.3
Total Capitalization	1,689.8	1,659.6
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, 2025 and 2024)	242.9	279.4
Current Liabilities:
Current portion of long-term debt	0.0	35.0
Advance customer billings	13.1	10.9
Wages and compensation accrued	7.8	7.2
Customer deposits	22.3	20.8
Taxes accrued	33.1	34.8
Regulatory liabilities	20.3	33.8
Other	14.6	13.8
Total Current Liabilities	290.0	249.4
Deferred Credits and Other Liabilities:
Deferred income taxes	64.1	35.9
Pension and postretirement benefit costs	2.4	28.0
Asset retirement obligations	470.0	468.6
Regulatory liabilities	29.2	28.3
Other	24.8	26.5
Total Deferred Credits and Other Liabilities	590.5	587.3
Commitments and Contingencies (Note 16)
Total Capitalization and Liabilities	2,570.3	2,496.3
Spire Alabama Inc [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	34.8	35.5
Current Liabilities:
Accounts payable	43.3	38.0
Spire Alabama Inc [Member] | Related Party [Member]
Accounts receivable:
Utility	0.0	0.4
Current Liabilities:
Notes payable	130.1	48.4
Accounts payable	$ 5.4	$ 6.7