Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 530.6	$ 573.8
Total operating revenues	36.3	28.2
Total operating revenues	566.9	602.0
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(7.2)	(4.4)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	530.6	573.8
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	530.7	575.2
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.1	1.4
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	32.3	25.8
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	32.3	25.8
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	566.9	602.0
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	574.1	606.4
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(7.2)	(4.4)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	533.7	581.0
Changes in accrued revenue under alternative revenue programs	(3.0)	(5.8)
Total operating revenues	530.7	575.2
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	368.4	393.6
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	122.5	130.3
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	31.0	29.5
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.4	14.9
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	2.4	12.7
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	32.3	25.8
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	32.3	25.8
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 11.1	$ 5.4