Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
ASSETS
Utility Plant	$ 9,490.1	$ 9,333.9	$ 8,946.3
Less: Accumulated depreciation and amortization	2,628.2	2,577.4	2,570.3
Net Utility Plant	6,861.9	6,756.5	6,376.0
Non-utility Property (net of accumulated depreciation and amortization of $139.2, $129.4 and $103.9 at December 31, 2025, September 30, 2025, and December 31, 2024, respectively)	1,003.4	1,007.2	982.5
Other Investments	125.4	128.0	118.5
Total Other Property and Investments	1,128.8	1,135.2	1,101.0
Current Assets:
Cash and cash equivalents	4.1	5.7	11.5
Accounts receivable:
Utility	426.2	191.9	379.6
Other	198.8	152.7	172.3
Allowance for credit losses	(28.7)	(28.8)	(34.7)
Delayed customer billings	18.5	13.6	18.4
Inventories:
Natural gas	203.4	226.9	187.0
Propane gas	8.6	8.6	8.6
Materials and supplies	47.5	47.0	47.0
Regulatory assets	63.4	78.3	89.6
Prepayments	34.5	47.8	41.5
Other	62.2	64.0	67.2
Total Current Assets	1,038.5	807.7	988.0
Deferred Charges and Other Assets:
Goodwill	1,171.6	1,171.6	1,171.6
Regulatory assets	1,331.4	1,323.5	1,288.4
Other	349.7	380.8	350.8
Total Deferred Charges and Other Assets	2,852.7	2,875.9	2,810.8
Total Assets	11,881.9	11,575.3	11,275.8
Capitalization:
Common stock (par value $1.00 per share; 70.0 million shares authorized; 59.1 million, 59.0 million, and 58.3 million shares issued and outstanding at December 31, 2025, September 30, 2025, and December 31, 2024, respectively)	59.1	59.0	58.3
Paid-in capital	1,981.8	1,981.4	1,933.7
Retained earnings	1,127.7	1,087.6	1,050.5
Accumulated other comprehensive income (loss)	22.1	19.4	24.4
Total Shareholders' Equity	3,432.7	3,389.4	3,308.9
Temporary equity	5.9	6.1	8.4
Long-term debt (less current portion)	4,449.4	3,369.4	3,697.7
Total Capitalization	7,888.0	6,764.9	7,015.0
Current Liabilities:
Current portion of long-term debt	488.1	487.5	42.5
Notes payable	412.0	1,317.0	1,158.0
Accounts payable	309.5	248.3	292.3
Advance customer billings	48.9	58.1	59.2
Wages and compensation accrued	29.7	54.1	28.7
Customer deposits	34.1	32.8	31.6
Taxes accrued	63.5	109.1	61.4
Regulatory liabilities	34.4	39.4	51.0
Other	293.1	202.3	266.5
Total Current Liabilities	1,713.3	2,548.6	1,991.2
Deferred Credits and Other Liabilities:
Deferred income taxes	914.1	887.4	838.3
Pension and postretirement benefit costs	47.7	74.7	126.6
Asset retirement obligations	589.5	583.2	586.0
Regulatory liabilities	587.6	578.0	577.2
Other	141.7	138.5	141.5
Total Deferred Credits and Other Liabilities	2,280.6	2,261.8	2,269.6
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	11,881.9	11,575.3	11,275.8
Depositary Shares [Member]
Capitalization:
Preferred stock ($25.00 par value per share; 10.0 million depositary shares authorized, issued and outstanding at December 31, 2025, September 30, 2025, and December 31, 2024)	242.0	242.0	242.0
Spire Missouri [Member]
ASSETS
Utility Plant	5,982.3	5,864.3	5,555.5
Less: Accumulated depreciation and amortization	1,139.2	1,104.6	1,104.5
Net Utility Plant	4,843.1	4,759.7	4,451.0
Total Other Property and Investments	73.8	75.8	72.9
Accounts receivable:
Other	28.5	34.0	23.8
Allowance for credit losses	(22.5)	(23.0)	(27.9)
Delayed customer billings	9.0	4.2	9.1
Inventories:
Natural gas	135.6	150.0	120.1
Propane gas	8.6	8.6	8.6
Materials and supplies	25.2	24.5	24.3
Regulatory assets	34.8	48.9	56.7
Prepayments	22.3	28.2	23.2
Other	0.4	0.0	0.0
Total Current Assets	574.5	426.7	537.2
Deferred Charges and Other Assets:
Goodwill	210.2	210.2	210.2
Regulatory assets	655.1	653.2	619.7
Other	194.1	224.2	194.3
Total Deferred Charges and Other Assets	1,059.4	1,087.6	1,024.2
Total Assets	6,550.8	6,349.8	6,085.3
Capitalization:
Paid-in capital and common stock	929.3	929.3	887.1
Retained earnings	1,311.3	1,239.1	1,164.2
Accumulated other comprehensive income (loss)	(0.7)	(0.7)	(1.9)
Total Shareholders' Equity	2,239.9	2,167.7	2,049.4
Long-term debt (less current portion)	2,152.7	1,953.6	1,803.6
Total Capitalization	4,392.6	4,121.3	3,853.0
Current Liabilities:
Advance customer billings	34.1	43.2	42.4
Wages and compensation accrued	17.4	27.0	16.2
Customer deposits	7.3	7.2	6.5
Taxes accrued	27.4	65.8	25.3
Regulatory liabilities	10.5	13.9	17.4
Other	127.2	64.3	104.6
Total Current Liabilities	827.7	908.0	908.6
Deferred Credits and Other Liabilities:
Deferred income taxes	617.9	598.7	581.7
Pension and postretirement benefit costs	47.4	72.4	91.1
Asset retirement obligations	96.5	95.5	96.6
Regulatory liabilities	490.9	481.3	486.0
Other	77.8	72.6	68.3
Total Deferred Credits and Other Liabilities	1,330.5	1,320.5	1,323.7
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	6,550.8	6,349.8	6,085.3
Spire Missouri [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	331.6	149.1	296.5
Current Liabilities:
Accounts payable	121.5	98.9	107.3
Spire Missouri [Member] | Related Party [Member]
Accounts receivable:
Utility	1.0	2.2	2.8
Current Liabilities:
Notes payable	465.7	566.3	580.9
Accounts payable	16.6	21.4	8.0
Spire Alabama Inc [Member]
ASSETS
Utility Plant	3,088.2	3,052.9	2,995.8
Less: Accumulated depreciation and amortization	1,379.8	1,362.4	1,353.5
Net Utility Plant	1,708.4	1,690.5	1,642.3
Total Other Property and Investments	0.1	0.1	0.1
Current Assets:
Cash and cash equivalents	0.0	1.9	0.0
Accounts receivable:
Other	7.0	6.9	5.6
Allowance for credit losses	(5.1)	(4.9)	(6.0)
Delayed customer billings	9.0	9.0	8.6
Inventories:
Natural gas	32.6	39.4	31.7
Materials and supplies	18.8	19.0	19.0
Regulatory assets	15.0	16.1	17.4
Prepayments	4.3	7.7	8.3
Other	0.3	0.0	0.0
Total Current Assets	158.4	129.9	150.4
Deferred Charges and Other Assets:
Regulatory assets	657.0	650.6	647.3
Other	99.4	99.2	95.0
Total Deferred Charges and Other Assets	756.4	749.8	742.3
Total Assets	2,623.3	2,570.3	2,535.1
Capitalization:
Paid-in capital and common stock	242.9	242.9	279.4
Retained earnings	738.8	735.2	677.1
Total Shareholders' Equity	981.7	978.1	956.5
Long-term debt (less current portion)	711.8	711.7	711.4
Total Capitalization	1,693.5	1,689.8	1,667.9
Current Liabilities:
Current portion of long-term debt	0.0	0.0	35.0
Advance customer billings	13.1	13.1	14.6
Wages and compensation accrued	4.8	7.8	4.4
Customer deposits	23.3	22.3	21.9
Taxes accrued	30.7	33.1	31.2
Regulatory liabilities	18.2	20.3	27.7
Other	15.8	14.6	17.4
Total Current Liabilities	330.3	290.0	273.1
Deferred Credits and Other Liabilities:
Deferred income taxes	70.6	64.1	41.0
Pension and postretirement benefit costs	0.5	2.4	26.7
Asset retirement obligations	475.1	470.0	473.7
Regulatory liabilities	28.8	29.2	26.9
Other	24.5	24.8	25.8
Total Deferred Credits and Other Liabilities	599.5	590.5	594.1
Total Capitalization and Liabilities	2,623.3	2,570.3	2,535.1
Spire Alabama Inc [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	76.4	34.8	65.3
Current Liabilities:
Accounts payable	53.9	43.3	46.7
Spire Alabama Inc [Member] | Related Party [Member]
Accounts receivable:
Utility	0.1	0.0	0.5
Current Liabilities:
Notes payable	164.6	130.1	67.0
Accounts payable	$ 5.9	$ 5.4	$ 7.2