Derivative Instruments and Hedging Activities - Open NYMEX Natural Gas Futures Positions (Details) - Spire Missouri - Natural Gas Futures Purchased - MMBTU MMBTU in Millions	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Derivative [Line Items]
Notional (MMBtu millions)	52.8	25.9
Maximum Term (Months)	12 months	12 months

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration