Note 6 - Long-term Debt - Composition of Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Total principal of long-term debt	$ 3,771.1	$ 3,732.7
Less: Unamortized discounts and debt issuance costs	(24.7)	(22.1)
Less: Current portion	(42.0)	(156.6)
Long-term debt (less current portion)	3,704.4	3,554.0
Spire Missouri [Member]
Total principal of long-term debt	1,818.0	1,798.0
Less: Unamortized discounts and debt issuance costs	(14.6)	(12.6)
Less: Current portion	(35.0)	0.0
Long-term debt (less current portion)	1,803.4	1,785.4
Spire Alabama Inc [Member]
Total principal of long-term debt	750.0	750.0
Less: Unamortized discounts and debt issuance costs	(3.7)	(4.1)
Long-term debt (less current portion)	711.3	745.9
Floating Rate Series Due December 2, 2024 [Member] | Spire Missouri [Member]
Total principal of long-term debt	0.0	300.0
First Mortgage Bonds Three Point Four Percentage Series Due August152023 [Member] | Spire Missouri [Member]
Total principal of long-term debt	45.0	45.0
First Mortgage Bonds Seven Percentage Series Due June12029 [Member] | Spire Missouri [Member]
Total principal of long-term debt	19.3	19.3
First Mortgage Bonds Two Point Eight Four Percentage Series Due November152029 [Member] | Spire Missouri [Member]
Total principal of long-term debt	275.0	275.0
First Mortgage Bonds Seven Point Nine Percentage Series Due September152030 [Member] | Spire Missouri [Member]
Total principal of long-term debt	30.0	30.0
First Mortgage Bonds Three Point Six Eight Percentage Series Due September152032 [Member] | Spire Missouri [Member]
Total principal of long-term debt	50.0	50.0
First Mortgage Bonds Four Point Eight Zero Percentage Series Due February 152033 [Member] | Spire Missouri [Member]
Total principal of long-term debt	400.0	400.0
First Mortgage Bonds Six Percentage Series Due May12034 [Member] | Spire Missouri [Member]
Total principal of long-term debt	99.3	99.3
First Mortgage Bonds Five Point One Five Percentage Series Due August152034 [Member] | Spire Missouri [Member]
Total principal of long-term debt	320.0	0.0
First Mortgage Bonds Six Point One Five Percentage Series Due June12036 [Member] | Spire Missouri [Member]
Total principal of long-term debt	54.5	54.5
First Mortgage Bonds Four Point Six Three Percentage Series Due August152043 [Member] | Spire Missouri [Member]
Total principal of long-term debt	99.9	99.9
First Mortgage Bonds Four Point Two Three Percentage Series Due September152047 [Member] | Spire Missouri [Member]
Total principal of long-term debt	70.0	70.0
First Mortgage Bonds Three Point Three Zero Percentage Series Due June12051 [Member] | Spire Missouri [Member]
Total principal of long-term debt	305.0	305.0
First Mortgage Bonds Four Point Three Eight Percentage Series Due September152057 [Member] | Spire Missouri [Member]
Total principal of long-term debt	50.0	50.0
Three Point Two One Percentage Notes Due September152025 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	35.0	35.0
Five Point Three Two Percentage Notes Due October152029 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	90.0	90.0
Two Point Eight Eight Percentage Notes Due December12029 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	100.0	100.0
Two Point Zero Four Percentage Notes Due December152030 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	150.0	150.0
Five Point Four One Percentage Notes Due October152032[Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	85.0	85.0
Five Point Nine Zero Percentage Notes Due January152037 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	45.0	45.0
Four Point Three One Percentage Notes Due December12045 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	80.0	80.0
Three Point Nine Two Percentage Notes Due January152048 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	45.0	45.0
Four Point Six Four Percentage Notes Due January152049 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	90.0	90.0
Four Point Zero Two Percentage Notes Due January152058 [Member] | Spire Alabama Inc [Member]
Total principal of long-term debt	30.0	30.0
Parent Company [Member] | Three Point Five Four Percent Senior Notes Due February272024 [Member]
Total principal of long-term debt	0.0	150.0
Parent Company [Member] | Zero Point Seven Five Percent Remarketable Senior Notes Due March12026 [Member]
Total principal of long-term debt	0.0	175.0
Parent Company [Member] | Five Point Three Zero Percent Senior Notes Due March 1, 2026 [Member]
Total principal of long-term debt	350.0	0.0
Parent Company [Member] | Three Point One Three Percent Senior Notes Due September12026 [Member]
Total principal of long-term debt	130.0	130.0
Parent Company [Member] | Three Point Nine Three Percent Senior Notes Due March152027 [Member]
Total principal of long-term debt	100.0	100.0
Parent Company [Member] | Five Point Eight Percent Senior Notes Due March152033 [Member]
Total principal of long-term debt	150.0	150.0
Parent Company [Member] | Four Point Seven Zero Percent Senior Notes Due August152044 [Member]
Total principal of long-term debt	250.0	250.0
Subsidiaries [Member]
Total principal of long-term debt	223.1
Subsidiaries [Member] | Five Point Zero Zero Percent First Mortgage Bonds Due September302031 [Member]
Total principal of long-term debt	42.0	42.0
Subsidiaries [Member] | Two Nine Five Percent Notes With Annual Principal Payments Through December2034 [Member]
Total principal of long-term debt	111.1	117.7
Subsidiaries [Member] | Five Point Six One Percent First Mortgage Bonds Due October 152037 [Member]
Total principal of long-term debt	30.0	30.0
Subsidiaries [Member] | Three Point Five Two Percent First Mortgage Bonds Due September302049 [Member]
Total principal of long-term debt	$ 40.0	$ 40.0