Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 776.7	$ 790.6	$ 1,350.5	$ 1,332.5
Total operating revenues	26.8	22.8	55.0	42.7
Total operating revenues	803.5	813.4	1,405.5	1,375.2
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(3.1)	(3.1)	(7.5)	(5.7)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues	776.7	790.6	1,350.5	1,332.5
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	776.8	790.8	1,352.0	1,332.8
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.1	0.2	1.5	0.3
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.4	21.3	51.2	40.9
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.5	21.3	51.3	$ 40.9
Gas Marketing | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.1	$ 0.0	0.1
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues	803.5		1,405.5
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	806.6		1,413.0
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues	(3.1)		(7.5)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	790.9		1,371.9
Changes in accrued revenue under alternative revenue programs	(14.1)		(19.9)
Total operating revenues	776.8		1,352.0
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	551.3		944.9
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	185.9		316.2
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	31.3		60.8
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	11.5		26.4
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	10.9		23.6
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.5		51.3
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues	25.5		51.3
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 4.3		$ 9.7