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Long-term Debt - Schedule of Composition of Long-term Debt (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
--- | --- | ---
Long-term Debt |  |
Total principal of long-term debt | $ 2,500.0 | $ 2,137.0
Less: Unamortized discounts and debt issuance costs | (15.9) | (14.4)
Less: Current portion | (60.4) | (40.0)
Long-term debt, excluding current portion | 2,423.7 | 2,082.6
Spire Missouri |  |
Long-term Debt |  |
Total principal of long-term debt | 1,098.0 | 930.0
Less: Unamortized discounts and debt issuance costs | (6.0) | (5.0)
Long-term debt, excluding current portion | 1,092.0 | 925.0
Spire Missouri | Floating-rate note due December 1, 2021 |  |
Long-term Debt |  |
Total principal of long-term debt |  | 100.0
Spire Missouri | 3.00% Series, due March 15, 2023 |  |
Long-term Debt |  |
Total principal of long-term debt | 55.0 | 55.0
Spire Missouri | 3.40% Series, due August 15, 2023 |  |
Long-term Debt |  |
Total principal of long-term debt | 250.0 | 250.0
Spire Missouri | 3.40% Series, due March 15, 2028 |  |
Long-term Debt |  |
Total principal of long-term debt | 45.0 | 45.0
Spire Missouri | 7.0% Series, due June 1, 2029 |  |
Long-term Debt |  |
Total principal of long-term debt | 19.3 | 25.0
Spire Missouri | 2.84% Series, due November 15, 2029 |  |
Long-term Debt |  |
Total principal of long-term debt | 275.0 |
Spire Missouri | 7.90% Series, due September 15, 2030 |  |
Long-term Debt |  |
Total principal of long-term debt | 30.0 | 30.0
Spire Missouri | 3.68% Series, due September 15, 2032 |  |
Long-term Debt |  |
Total principal of long-term debt | 50.0 | 50.0
Spire Missouri | 6.00% Series, due May 1, 2034 |  |
Long-term Debt |  |
Total principal of long-term debt | 99.3 | 100.0
Spire Missouri | 6.15% Series, due June 1, 2036 |  |
Long-term Debt |  |
Total principal of long-term debt | 54.5 | 55.0
Spire Missouri | 4.63% Series, due August 15, 2043 |  |
Long-term Debt |  |
Total principal of long-term debt | 99.9 | 100.0
Spire Missouri | 4.23% Series, due September 15, 2047 |  |
Long-term Debt |  |
Total principal of long-term debt | 70.0 | 70.0
Spire Missouri | 4.38% Series, due September 15, 2057 |  |
Long-term Debt |  |
Total principal of long-term debt | 50.0 | 50.0
Spire Alabama |  |
Long-term Debt |  |
Total principal of long-term debt | 475.0 | 415.0
Less: Unamortized discounts and debt issuance costs | (3.2) | (2.8)
Less: Current portion |  | (40.0)
Long-term debt, excluding current portion | 471.8 | 372.2
Spire Alabama | 5.20% Notes, due January 15, 2020 |  |
Long-term Debt |  |
Total principal of long-term debt |  | 40.0
Spire Alabama | 3.86% Notes, due December 22, 2021 |  |
Long-term Debt |  |
Total principal of long-term debt | 50.0 | 50.0
Spire Alabama | 3.21% Notes, due September 15, 2025 |  |
Long-term Debt |  |
Total principal of long-term debt | 35.0 | 35.0
Spire Alabama | 2.88% Notes, due December 1, 2029 |  |
Long-term Debt |  |
Total principal of long-term debt | 100.0 |
Spire Alabama | 5.90% Notes, due January 15, 2037 |  |
Long-term Debt |  |
Total principal of long-term debt | 45.0 | 45.0
Spire Alabama | 4.31% Notes, due December 1, 2045 |  |
Long-term Debt |  |
Total principal of long-term debt | 80.0 | 80.0
Spire Alabama | 3.92% Notes, due January 15, 2048 |  |
Long-term Debt |  |
Total principal of long-term debt | 45.0 | 45.0
Spire Alabama | 4.64% Notes, due January 15, 2049 |  |
Long-term Debt |  |
Total principal of long-term debt | 90.0 | 90.0
Spire Alabama | 4.02% Notes, due January 15, 2058 |  |
Long-term Debt |  |
Total principal of long-term debt | 30.0 | 30.0
Spire | 2.52% Senior Notes, due September 1, 2021 |  |
Long-term Debt |  |
Total principal of long-term debt | 35.0 | 35.0
Spire | 3.31% Notes Payable, due December 15, 2022 |  |
Long-term Debt |  |
Total principal of long-term debt | 25.0 | 25.0
Spire | 3.54% Senior Notes, due February 27, 2024 |  |
Long-term Debt |  |
Total principal of long-term debt | 150.0 | 150.0
Spire | 3.13% Senior Notes, due September 1, 2026 |  |
Long-term Debt |  |
Total principal of long-term debt | 130.0 | 130.0
Spire | 3.93% Senior Notes, due March 15, 2027 |  |
Long-term Debt |  |
Total principal of long-term debt | 100.0 | 100.0
Spire | 4.70% Senior Notes, due August 15, 2044 |  |
Long-term Debt |  |
Total principal of long-term debt | 250.0 | 250.0
Subsidiaries |  |
Long-term Debt |  |
Total principal of long-term debt | 237.0 |
Subsidiaries | 4.14% First Mortgage Bonds, due September 30, 2021 |  |
Long-term Debt |  |
Total principal of long-term debt | 20.0 | 20.0
Subsidiaries | 5.00% First Mortgage Bonds, due September 30, 2031 |  |
Long-term Debt |  |
Total principal of long-term debt | 42.0 | 42.0
Subsidiaries | 2.95% Notes, with Annual Principal Payments through December 2034 |  |
Long-term Debt |  |
Total principal of long-term debt | 135.0 |
Subsidiaries | 3.52% First Mortgage Bonds, due September 30, 2049 |  |
Long-term Debt |  |
Total principal of long-term debt | $ 40.0 | $ 40.0

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