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

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 210
-SubTopic 10
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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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