Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024
ASSETS
Utility Plant	$ 9,080.6	$ 8,779.1	$ 8,480.3
Less: Accumulated depreciation and amortization	2,575.1	2,535.8	2,509.3
Net Utility Plant	6,505.5	6,243.3	5,971.0
Non-utility Property (net of accumulated depreciation and amortization of $111.1, $96.8 and $115.9 at March 31, 2025, September 30, 2024, and March 31, 2024, respectively)	1,005.7	955.3	886.2
Other Investments	117.9	115.3	105.3
Total Other Property and Investments	1,123.6	1,070.6	991.5
Current Assets:
Cash and cash equivalents	15.2	4.5	25.6
Accounts receivable:
Utility	373.4	196.3	398.5
Other	193.3	112.5	107.3
Allowance for credit losses	(36.8)	(31.4)	(39.1)
Delayed customer billings	37.4	12.0	54.8
Inventories:
Natural gas	124.6	208.6	159.0
Propane gas	8.6	8.6	8.6
Materials and supplies	45.9	46.7	47.2
Regulatory assets	42.3	115.4	116.9
Prepayments	39.2	47.6	34.1
Other	64.7	50.5	92.8
Total Current Assets	907.8	771.3	1,005.7
Deferred Charges and Other Assets:
Goodwill	1,171.6	1,171.6	1,171.6
Regulatory assets	1,287.2	1,251.8	1,272.7
Other	351.0	352.1	298.9
Total Deferred Charges and Other Assets	2,809.8	2,775.5	2,743.2
Total Assets	11,346.7	10,860.7	10,711.4
Capitalization:
Common stock (par value $1.00 per share; 70.0 million shares authorized; 59.0 million, 57.7 million, and 57.7 million shares issued and outstanding at March 31, 2025, September 30, 2024, and March 31, 2024, respectively)	59.0	57.7	57.7
Paid-in capital	1,977.4	1,902.2	1,899.7
Retained earnings	1,207.6	1,018.7	1,155.3
Accumulated other comprehensive income (loss)	22.7	12.1	35.6
Total Shareholders' Equity	3,508.7	3,232.7	3,390.3
Temporary equity	9.3	8.6	10.3
Long-term debt (less current portion)	3,348.5	3,704.4	3,421.4
Total Capitalization	6,866.5	6,945.7	6,822.0
Current Liabilities:
Current portion of long-term debt	392.5	42.0	307.0
Notes payable	1,015.0	947.0	786.0
Accounts payable	283.5	237.2	193.4
Advance customer billings	16.1	48.4	6.1
Wages and compensation accrued	41.2	51.5	40.2
Customer deposits	32.2	29.9	29.4
Taxes accrued	81.0	105.2	82.6
Regulatory liabilities	51.6	49.5	25.1
Other current liabilities	199.4	193.2	180.5
Total Current Liabilities	2,112.5	1,703.9	1,650.3
Deferred Credits and Other Liabilities:
Deferred income taxes	890.7	808.4	816.6
Pension and postretirement benefit costs	110.8	146.7	130.0
Asset retirement obligations	592.3	579.9	589.7
Regulatory liabilities	637.0	535.5	557.7
Other noncurrent liabilities	136.9	140.6	145.1
Total Deferred Credits and Other Liabilities	2,367.7	2,211.1	2,239.1
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	11,346.7	10,860.7	10,711.4
Depositary Shares [Member]
Capitalization:
Preferred stock ($25.00 par value per share; 10.0 million depositary shares authorized, issued and outstanding at March 31, 2025, September 30, 2024, and March 31, 2024)	242.0	242.0	242.0
Spire Missouri [Member]
ASSETS
Utility Plant	5,663.3	5,420.2	5,193.5
Less: Accumulated depreciation and amortization	1,094.0	1,086.0	1,088.1
Net Utility Plant	4,569.3	4,334.2	4,105.4
Total Other Property and Investments	71.3	70.1	67.8
Accounts receivable:
Other	42.9	22.2	23.8
Allowance for credit losses	(29.6)	(24.9)	(32.0)
Delayed customer billings	34.7	5.7	53.6
Inventories:
Natural gas	80.0	129.6	101.9
Propane gas	8.6	8.6	8.6
Materials and supplies	23.5	24.4	24.0
Regulatory assets	15.5	84.0	89.2
Prepayments	21.9	27.2	17.0
Total Current Assets	468.6	432.5	595.8
Deferred Charges and Other Assets:
Goodwill	210.2	210.2	210.2
Regulatory assets	613.8	588.0	630.5
Other	194.2	193.6	145.2
Total Deferred Charges and Other Assets	1,018.2	991.8	985.9
Total Assets	6,127.4	5,828.6	5,754.9
Capitalization:
Paid-in capital and common stock	929.3	854.9	854.9
Retained earnings	1,277.1	1,110.8	1,154.6
Accumulated other comprehensive income (loss)	(1.4)	(2.0)	(2.4)
Total Shareholders' Equity	2,205.0	1,963.7	2,007.1
Long-term debt (less current portion)	1,803.8	1,803.4	1,486.2
Total Capitalization	4,008.8	3,767.1	3,493.3
Current Liabilities:
Current portion of long-term debt	0.0	0.0	300.0
Advance customer billings	8.4	35.5	0.0
Wages and compensation accrued	21.4	24.2	19.9
Customer deposits	6.5	6.1	5.9
Taxes accrued	47.7	60.2	47.0
Regulatory liabilities	12.0	10.2	0.0
Other current liabilities	45.6	50.6	44.6
Total Current Liabilities	716.3	781.7	949.5
Deferred Credits and Other Liabilities:
Deferred income taxes	603.1	567.6	563.9
Pension and postretirement benefit costs	89.5	110.0	100.5
Asset retirement obligations	97.6	95.7	113.4
Regulatory liabilities	546.1	443.3	475.3
Other noncurrent liabilities	66.0	63.2	59.0
Total Deferred Credits and Other Liabilities	1,402.3	1,279.8	1,312.1
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	6,127.4	5,828.6	5,754.9
Spire Missouri [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	268.2	152.9	307.6
Current Liabilities:
Accounts payable	104.5	92.0	78.7
Spire Missouri [Member] | Related Party [Member]
Accounts receivable:
Utility	2.9	2.8	2.1
Current Liabilities:
Notes payable	461.3	495.3	243.9
Accounts payable	8.9	7.6	9.5
Spire Alabama Inc [Member]
ASSETS
Utility Plant	3,019.5	2,966.6	2,900.7
Less: Accumulated depreciation and amortization	1,369.2	1,336.6	1,307.4
Net Utility Plant	1,650.3	1,630.0	1,593.3
Total Other Property and Investments	0.1	0.1	0.0
Current Assets:
Cash and cash equivalents	3.3	1.5	1.8
Accounts receivable:
Other	5.9	6.1	6.8
Allowance for credit losses	(6.1)	(5.7)	(6.2)
Delayed customer billings	2.5	5.7	1.0
Inventories:
Natural gas	28.5	37.3	34.1
Materials and supplies	18.5	18.7	19.2
Regulatory assets	13.8	19.2	14.9
Prepayments	6.0	10.7	3.3
Total Current Assets	158.1	129.4	148.9
Deferred Charges and Other Assets:
Regulatory assets	652.5	642.0	619.1
Other	94.9	94.8	84.2
Total Deferred Charges and Other Assets	747.4	736.8	703.3
Total Assets	2,555.9	2,496.3	2,445.5
Capitalization:
Paid-in capital and common stock	249.4	279.4	279.4
Retained earnings	740.6	668.9	687.5
Total Shareholders' Equity	990.0	948.3	966.9
Long-term debt (less current portion)	711.5	711.3	746.1
Total Capitalization	1,701.5	1,659.6	1,713.0
Current Liabilities:
Current portion of long-term debt	35.0	35.0	0.0
Advance customer billings	6.5	10.9	4.7
Wages and compensation accrued	6.1	7.2	5.6
Customer deposits	22.4	20.8	20.5
Taxes accrued	27.1	34.8	28.8
Regulatory liabilities	33.9	33.8	18.7
Other current liabilities	14.2	13.8	14.1
Total Current Liabilities	247.2	249.4	163.6
Deferred Credits and Other Liabilities:
Deferred income taxes	64.4	35.9	36.8
Pension and postretirement benefit costs	12.4	28.0	22.5
Asset retirement obligations	478.7	468.6	460.6
Regulatory liabilities	26.4	28.3	20.1
Other noncurrent liabilities	25.3	26.5	28.9
Total Deferred Credits and Other Liabilities	607.2	587.3	568.9
Total Capitalization and Liabilities	2,555.9	2,496.3	2,445.5
Spire Alabama Inc [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	85.7	35.5	74.0
Current Liabilities:
Accounts payable	49.9	38.0	28.4
Spire Alabama Inc [Member] | Related Party [Member]
Accounts receivable:
Utility	0.0	0.4	0.0
Current Liabilities:
Notes payable	41.8	48.4	37.4
Accounts payable	$ 10.3	$ 6.7	$ 5.4