Leases - Schedule of Maturity Analysis for Operating Lease Liabilities (Details) $ in Millions	Mar. 31, 2020 USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Remainder of 2020	$ 3.4
2021	7.2
2022	7.2
2023	7.2
2024	5.8
2025	5.1
Thereafter	58.4
Total undiscounted lease payments	94.3
Less present value discount	(26.8)
Total current and noncurrent lease liabilities	67.5
Spire Missouri Inc
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Remainder of 2020	0.1
2021	0.4
2022	0.4
2023	0.3
2024	0.3
2025	0.3
Thereafter	0.2
Total undiscounted lease payments	2.0
Less present value discount	(0.2)
Total current and noncurrent lease liabilities	1.8
Spire Alabama
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Remainder of 2020	1.2
2021	2.1
2022	2.1
2023	2.1
2024	0.7
Total undiscounted lease payments	8.2
Less present value discount	(0.3)
Total current and noncurrent lease liabilities	$ 7.9

Lessee operating lease liability payments due after year six.

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- Definition

Lessee operating lease liability payments due year six.

+ References

No definition available.

+ Details

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- Definition

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

+ References

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=77888426&loc=SL77918701-209980

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- Definition

Amount of lessee's undiscounted obligation for lease payments for operating lease, due in fifth fiscal year following latest fiscal year.

+ References

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Amount of lessee's undiscounted obligation for lease payments for operating lease, due in fourth fiscal year following latest fiscal year.

+ References

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Amount of lessee's undiscounted obligation for lease payments for operating lease, due in third fiscal year following latest fiscal year.

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payments for operating lease, due in second fiscal year following latest fiscal year.

+ References

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- Definition

Amount of lessee's undiscounted obligation for lease payments for operating lease having an initial or remaining lease term in excess of one year due in remainder of fiscal year following latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918701-209980

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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.

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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=77888419&loc=SL77918627-209977

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