Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 573.8	$ 541.9
Total operating revenues	28.2	19.9
Total operating revenues	602.0	561.8
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(4.4)	(2.6)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	573.8	541.9
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	575.2	542.0
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	1.4	0.1
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.8	19.6
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.8	19.6
Gas Marketing | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.0	$ 0.0
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	602.0
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	606.4
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(4.4)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	581.0
Changes in accrued revenue under alternative revenue programs	(5.8)
Total operating revenues	575.2
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	393.6
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	130.3
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	29.5
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	14.9
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	12.7
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.8
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.8
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 5.4