Derivative Instruments and Hedging Activities - Fair Value of Derivative Instruments in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Fair value of derivative assets presented above	$ 24.2	$ 27.4
Fair value of derivative liabilities presented above	78.5	21.0
Not Designated as Hedging Instrument
Derivative [Line Items]
Fair value of derivative assets presented above	24.2	24.4
Fair value of derivative liabilities presented above	35.1	21.0
Spire Missouri
Derivative [Line Items]
Fair value of derivative assets presented above	0.0	2.7
Fair value of derivative liabilities presented above	12.3	1.9
Spire Missouri \| Not Designated as Hedging Instrument \| Accounts Receivable – Other \| Gas Utility \| Natural Gas Contracts
Derivative [Line Items]
Fair value of derivative assets presented above	0.0	2.7
Fair value of derivative liabilities presented above	$ 12.3	$ 1.9

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant