Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 301.4	$ 334.8	$ 1,651.9	$ 1,667.3
Total operating revenues	19.9	15.8	74.9	58.5
Total operating revenues	321.3	350.6	1,726.8	1,725.8
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(3.2)	(2.9)	(10.7)	(8.6)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	301.4	334.8	1,651.9	1,667.3
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	301.6	334.8	1,653.6	1,667.6
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.2	0.0	1.7	0.3
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	18.4	14.4	69.6	55.3
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	18.3	14.4	69.6	$ 55.3
Gas Marketing | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(0.1)	$ 0.0
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	321.3		1,726.8
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	324.5		1,737.5
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(3.2)		(10.7)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	298.7		1,670.6
Changes in accrued revenue under alternative revenue programs	2.9		(17.0)
Total operating revenues	301.6		1,653.6
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	188.1		1,133.0
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	68.5		384.7
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.6		86.4
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	7.7		34.1
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	8.8		32.4
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	18.3		69.6
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	18.3		69.6
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 4.6		$ 14.3