Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024
ASSETS
Utility Plant	$ 9,236.2	$ 8,779.1	$ 8,612.9
Less: Accumulated depreciation and amortization	2,571.8	2,535.8	2,510.4
Net Utility Plant	6,664.4	6,243.3	6,102.5
Non-utility Property (net of accumulated depreciation and amortization of $119.9, $96.8 and $90.1 at June 30, 2025, September 30, 2024, and June 30, 2024, respectively)	1,011.2	955.3	917.9
Other Investments	125.4	115.3	112.1
Total Other Property and Investments	1,136.6	1,070.6	1,030.0
Current Assets:
Cash and cash equivalents	13.1	4.5	7.4
Accounts receivable:
Utility	228.0	196.3	238.4
Other	163.8	112.5	113.4
Allowance for credit losses	(33.8)	(31.4)	(33.2)
Delayed customer billings	18.3	12.0	26.8
Inventories:
Natural gas	169.9	208.6	175.0
Propane gas	8.6	8.6	8.6
Materials and supplies	46.4	46.7	46.5
Regulatory assets	35.0	115.4	89.8
Prepayments	60.6	47.6	59.3
Other	70.2	50.5	93.8
Total Current Assets	780.1	771.3	825.8
Deferred Charges and Other Assets:
Goodwill	1,171.6	1,171.6	1,171.6
Regulatory assets	1,293.4	1,251.8	1,282.8
Other	350.2	352.1	298.2
Total Deferred Charges and Other Assets	2,815.2	2,775.5	2,752.6
Total Assets	11,396.3	10,860.7	10,710.9
Capitalization:
Common stock (par value $1.00 per share; 70.0 million shares authorized; 59.0 million, 59.0 million, 57.7 million, and 57.7 million shares issued and outstanding at June 30, 2025, September 30, 2024, and June 30, 2024, respectively)	59.0	57.7	57.7
Paid-in capital	1,979.2	1,902.2	1,901.5
Retained earnings	1,179.5	1,018.7	1,093.4
Accumulated other comprehensive income (loss)	20.5	12.1	38.6
Total Shareholders' Equity	3,480.2	3,232.7	3,333.2
Temporary equity	5.2	8.6	8.6
Long-term debt (less current portion)	3,498.4	3,704.4	3,422.3
Total Capitalization	6,983.8	6,945.7	6,764.1
Current Liabilities:
Current portion of long-term debt	392.5	42.0	307.0
Notes payable	1,009.5	947.0	771.0
Accounts payable	240.2	237.2	205.2
Advance customer billings	29.4	48.4	17.1
Wages and compensation accrued	39.6	51.5	41.5
Customer deposits	32.6	29.9	29.5
Taxes accrued	91.9	105.2	90.8
Regulatory liabilities	48.6	49.5	37.9
Other current liabilities	202.2	193.2	209.8
Total Current Liabilities	2,086.5	1,703.9	1,709.8
Deferred Credits and Other Liabilities:
Deferred income taxes	900.4	808.4	819.6
Pension and postretirement benefit costs	105.0	146.7	128.5
Asset retirement obligations	598.6	579.9	596.0
Regulatory liabilities	582.0	535.5	547.5
Other noncurrent liabilities	140.0	140.6	145.4
Total Deferred Credits and Other Liabilities	2,326.0	2,211.1	2,237.0
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	11,396.3	10,860.7	10,710.9
Depositary Shares [Member]
Capitalization:
Preferred stock ($25.00 par value per share; 10.0 million depositary shares authorized, issued and outstanding at June 30, 2025, September 30, 2024, and June 30, 2024)	242.0	242.0	242.0
Spire Missouri [Member]
ASSETS
Utility Plant	5,793.3	5,420.2	5,296.6
Less: Accumulated depreciation and amortization	1,089.8	1,086.0	1,073.5
Net Utility Plant	4,703.5	4,334.2	4,223.1
Total Other Property and Investments	73.6	70.1	69.4
Current Assets:
Cash and cash equivalents	0.0	0.0	1.3
Accounts receivable:
Other	33.0	22.2	18.4
Allowance for credit losses	(27.9)	(24.9)	(26.6)
Delayed customer billings	14.2	5.7	24.5
Inventories:
Natural gas	99.1	129.6	103.7
Propane gas	8.6	8.6	8.6
Materials and supplies	23.4	24.4	24.1
Regulatory assets	8.7	84.0	63.0
Prepayments	37.3	27.2	36.6
Total Current Assets	363.6	432.5	431.5
Deferred Charges and Other Assets:
Goodwill	210.2	210.2	210.2
Regulatory assets	613.5	588.0	634.1
Other	194.6	193.6	144.8
Total Deferred Charges and Other Assets	1,018.3	991.8	989.1
Total Assets	6,159.0	5,828.6	5,713.1
Capitalization:
Paid-in capital and common stock	929.3	854.9	854.9
Retained earnings	1,264.1	1,110.8	1,137.8
Accumulated other comprehensive income (loss)	(1.3)	(2.0)	(1.5)
Total Shareholders' Equity	2,192.1	1,963.7	1,991.2
Long-term debt (less current portion)	1,953.2	1,803.4	1,486.6
Total Capitalization	4,145.3	3,767.1	3,477.8
Current Liabilities:
Current portion of long-term debt	0.0	0.0	300.0
Advance customer billings	19.1	35.5	8.8
Wages and compensation accrued	19.8	24.2	18.0
Customer deposits	7.0	6.1	5.9
Taxes accrued	53.0	60.2	49.5
Regulatory liabilities	14.0	10.2	0.0
Other current liabilities	51.6	50.6	46.6
Total Current Liabilities	663.9	781.7	925.7
Deferred Credits and Other Liabilities:
Deferred income taxes	603.6	567.6	567.7
Pension and postretirement benefit costs	85.0	110.0	100.1
Asset retirement obligations	98.6	95.7	114.5
Regulatory liabilities	491.1	443.3	465.7
Other noncurrent liabilities	71.5	63.2	61.6
Total Deferred Credits and Other Liabilities	1,349.8	1,279.8	1,309.6
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	6,159.0	5,828.6	5,713.1
Spire Missouri [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	164.9	152.9	176.3
Current Liabilities:
Accounts payable	90.8	92.0	68.2
Spire Missouri [Member] | Related Party [Member]
Accounts receivable:
Utility	2.3	2.8	1.6
Current Liabilities:
Notes payable	398.7	495.3	418.3
Accounts payable	9.9	7.6	10.4
Spire Alabama Inc [Member]
ASSETS
Utility Plant	3,042.8	2,966.6	2,929.1
Less: Accumulated depreciation and amortization	1,370.8	1,336.6	1,324.9
Net Utility Plant	1,672.0	1,630.0	1,604.2
Total Other Property and Investments	0.1	0.1	0.0
Current Assets:
Cash and cash equivalents	1.7	1.5	2.3
Accounts receivable:
Other	6.2	6.1	5.3
Allowance for credit losses	(5.1)	(5.7)	(5.7)
Delayed customer billings	3.9	5.7	2.0
Inventories:
Natural gas	31.1	37.3	34.4
Materials and supplies	19.3	18.7	18.5
Regulatory assets	13.2	19.2	14.7
Prepayments	9.4	10.7	9.1
Total Current Assets	133.3	129.4	133.3
Deferred Charges and Other Assets:
Regulatory assets	659.6	642.0	626.0
Other	94.8	94.8	84.8
Total Deferred Charges and Other Assets	754.4	736.8	710.8
Total Assets	2,559.8	2,496.3	2,448.3
Capitalization:
Paid-in capital and common stock	242.9	279.4	279.4
Retained earnings	745.1	668.9	683.9
Total Shareholders' Equity	988.0	948.3	963.3
Long-term debt (less current portion)	711.6	711.3	746.2
Total Capitalization	1,699.6	1,659.6	1,709.5
Current Liabilities:
Current portion of long-term debt	35.0	35.0	0.0
Advance customer billings	8.4	10.9	6.5
Wages and compensation accrued	5.6	7.2	5.2
Customer deposits	22.3	20.8	20.5
Taxes accrued	30.6	34.8	32.7
Regulatory liabilities	29.3	33.8	32.2
Other current liabilities	11.9	13.8	12.2
Total Current Liabilities	248.1	249.4	167.3
Deferred Credits and Other Liabilities:
Deferred income taxes	65.9	35.9	37.8
Pension and postretirement benefit costs	11.1	28.0	21.2
Asset retirement obligations	483.9	468.6	465.6
Regulatory liabilities	25.8	28.3	19.6
Other noncurrent liabilities	25.4	26.5	27.3
Total Deferred Credits and Other Liabilities	612.1	587.3	571.5
Total Capitalization and Liabilities	2,559.8	2,496.3	2,448.3
Spire Alabama Inc [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	53.5	35.5	52.2
Current Liabilities:
Accounts payable	43.6	38.0	30.2
Spire Alabama Inc [Member] | Related Party [Member]
Accounts receivable:
Utility	0.1	0.4	0.5
Current Liabilities:
Notes payable	56.4	48.4	23.0
Accounts payable	$ 5.0	$ 6.7	$ 4.8