Note 6 - Long-term Debt (Details Textual) - USD ($) $ in Millions		12 Months Ended			19 Months Ended
	Dec. 07, 2021	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2023
Long-Term Debt, Gross		$ 3,732.7	$ 3,258.9		$ 3,732.7
Interest Paid, Capitalized, Investing Activities		8.6	4.5	$ 4.4
Stock Issued During Period, Value, New Issues		40.5	50.3
Subsidiaries [Member]
Long-Term Debt, Gross		229.7			229.7
Spire Missouri [Member]
Long-Term Debt, Gross		1,798.0	1,648.0		1,798.0
Interest Paid, Capitalized, Investing Activities		3.1	0.6	0.2
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized		800.0			800.0
Stock Issued During Period, Value, New Issues		38.7	51.1
Spire Missouri [Member] | Universal Shelf Registration [Member]
Stock Issued During Period, Value, New Issues		79.1			79.1
Spire Missouri [Member] | First Mortgage Bonds Due December Two Two Thousand Twenty Four [Member]
Long-Term Debt, Gross	$ 300.0
Debt Instrument, Interest Rate, Stated Percentage	8.00%
Spire Missouri [Member] | First Mortgage Bonds Due December Two Two Thousand Twenty Four [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Debt Instrument, Basis Spread on Variable Rate	0.50%
Spire Missouri [Member] | Floating Rate Series Due December 2, 2024 [Member]
Long-Term Debt, Gross		300.0			300.0
Spire Missouri [Member] | First Mortgage Bond [Member]
Debt Instrument, Face Amount		400.0			400.0
Spire Alabama Inc [Member]
Long-Term Debt, Gross		750.0	575.0		$ 750.0
Interest Paid, Capitalized, Investing Activities		$ 2.3	$ 3.2	$ 3.2