Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 566.9	$ 602.0
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	375.6	417.7
Changes in accrued revenue under alternative revenue programs	(1.6)	(4.5)
Total Operating Revenues	374.0	413.2
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	275.9	302.1
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	80.8	89.1
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.0	9.0
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.4	14.9
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.5	2.6
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	126.8	134.6
Changes in accrued revenue under alternative revenue programs	(0.6)	(1.1)
Total Operating Revenues	126.2	133.5
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	74.3	75.4
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	31.3	31.4
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	19.6	18.2
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 1.6	$ 9.6