Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 602.0	$ 561.8
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	417.7
Changes in accrued revenue under alternative revenue programs	(4.5)
Total Operating Revenues	413.2	392.3
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	302.1
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	89.1
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.0
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	14.9
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.6
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	134.6
Changes in accrued revenue under alternative revenue programs	(1.1)
Total Operating Revenues	133.5	$ 120.8
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	75.4
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	31.4
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	18.2
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 9.6