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) - 3 months ended Dec. 31, 2019 $ in Millions	USD ($)	reporting_unit	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	$ 3.3