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Note 6 - Long-term Debt - Composition of Long-term Debt (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Total principal of long-term debt | $ 3,732.7 | $ 3,258.9
Less: Unamortized discounts and debt issuance costs | (22.1) | (19.2)
Less: Current portion | (156.6) | (281.2)
Long-term debt (less current portion) | 3,554.0 | 2,958.5
Spire Missouri [Member] |  |
Total principal of long-term debt | 1,798.0 | 1,648.0
Less: Unamortized discounts and debt issuance costs | (12.6) | (10.3)
Less: Current portion | 0.0 | (250.0)
Long-term debt (less current portion) | 1,785.4 | 1,387.7
Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 750.0 | 575.0
Less: Unamortized discounts and debt issuance costs | (4.1) | (3.5)
Long-term debt (less current portion) | 745.9 | 571.5
First Mortgage Bonds Three Point Four Percentage Series Due August152023 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 0.0 | 250.0
Floating Rate Series Due December 2, 2024 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 300.0 |
First Mortgage Bonds Three Point Four Percentage Series Due March152028 [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 | 0.0
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 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 | 0.0
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 | 0.0
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 Three One Percent Notes Payable Due December152022 [Member] |  |
Total principal of long-term debt | 0.0 | 25.0
Parent Company [Member] | Three Point Five Four Percent Senior Notes Due February272024 [Member] |  |
Total principal of long-term debt | 150.0 | 150.0
Parent Company [Member] | Zero Point Seven Five Percent Remarketable Senior Notes Due March12026 [Member] |  |
Total principal of long-term debt | 175.0 | 175.0
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 | 0.0
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 | 229.7 |
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 | 117.7 | 123.9
Subsidiaries [Member] | Five Point Six One Percent First Mortgage Bonds Due October 152037 [Member] |  |
Total principal of long-term debt | 30.0 | 0.0
Subsidiaries [Member] | Three Point Five Two Percent First Mortgage Bonds Due September302049 [Member] |  |
Total principal of long-term debt | $ 40.0 | $ 40.0

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