Fair Value of Financial Instruments (Tables)

12 Months Ended

Sep. 30, 2025

Fair Value Disclosures [Abstract]

Schedule of Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis

The carrying amounts and estimated fair values of financial instruments not measured at fair value on a recurring basis were as follows:

					Classification of
					Estimated Fair Value
					Quoted
					Prices in		Significant
					Active		Observable
	Carrying		Fair		Markets		Inputs
	Amount		Value		(Level 1)		(Level 2)
Spire
As of September 30, 2025
Cash and cash equivalents	$	5.7	$	5.7	$	5.7	$	—
Notes payable		1,317.0		1,317.0		—		1,317.0
Long-term debt, including current portion		3,856.9		3,691.5		—		3,691.5
As of September 30, 2024
Cash and cash equivalents	$	4.5	$	4.5	$	4.5	$	—
Notes payable		947.0		947.0		—		947.0
Long-term debt, including current portion		3,746.4		3,600.3		—		3,600.3
Spire Missouri
As of September 30, 2025
Notes payable - associated companies	$	566.3	$	566.3	$	—	$	566.3
Long-term debt		1,953.6		1,874.0		—		1,874.0
As of September 30, 2024
Notes payable - associated companies	$	495.3	$	495.3	$	—	$	495.3
Long-term debt		1,803.4		1,736.9		—		1,736.9
Spire Alabama
As of September 30, 2025
Cash and cash equivalents	$	1.9	$	1.9	$	1.9	$	—
Notes payable - associated companies		130.1		130.1		—		130.1
Long-term debt		711.7		675.9		—		675.9
As of September 30, 2024
Cash and cash equivalents	$	1.5	$	1.5	$	1.5	$	—
Notes payable - associated companies		48.4		48.4		—		48.4
Long-term debt, including current portion		746.3		711.8		—		711.8