Leases - Schedule of Maturity Analysis for Operating Lease Liabilities (Details) $ in Millions	Sep. 30, 2020 USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021	$ 6.6
2022	7.2
2023	7.2
2024	5.8
2025	5.1
Thereafter	58.4
Total undiscounted lease payments	90.3
Less present value discount	(25.4)
Total current and noncurrent lease liabilities	64.9
Spire Missouri Inc
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021	0.3
2022	0.4
2023	0.3
2024	0.3
2025	0.3
Thereafter	0.2
Total undiscounted lease payments	1.8
Less present value discount	(0.1)
Total current and noncurrent lease liabilities	1.7
Spire Alabama
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021	1.9
2022	2.1
2023	2.1
2024	0.7
Total undiscounted lease payments	6.8
Less present value discount	(0.2)
Total current and noncurrent lease liabilities	$ 6.6

Amount of lessee's undiscounted obligation for lease payment for operating lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918701-209980

+ Details

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Balance Type:	credit
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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease due after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payment for operating lease to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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- Definition

Amount of lessee's undiscounted obligation for lease payments in excess of discounted obligation for lease payments for operating lease.

+ References

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No definition available.

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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

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