Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2020 reporting_unit	Mar. 31, 2020 operating-segment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Number of reportable segments			2	2
Affiliated Entity \| Spire Marketing \| Spire Alabama \| Regulated Operation
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Purchases of natural gas from related party	$ 1.0	$ 4.3