Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 251.9	$ 321.1	$ 715.5	$ 566.9	$ 225.6	$ 321.3	$ 803.5	$ 602.0	$ 1,855.4	$ 1,952.4	$ 1,965.0
Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues									(42.3)	(13.6)	(11.5)
Gas Utility
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,751.8	1,859.2	1,888.0
Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,752.0	1,860.8	1,888.4
Gas Utility | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues									0.2	1.6	0.4
Gas Marketing
Disaggregation Of Revenue [Line Items]
Total operating revenues									87.9	83.7	71.6
Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues									87.9	83.7	$ 71.6
Spire
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,855.4	1,952.4
Spire | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,897.7	1,966.0
Spire | Intersegment Eliminations
Disaggregation Of Revenue [Line Items]
Total operating revenues									(42.3)	(13.6)
Spire | Gas Utility | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,747.7	1,877.7
Changes in accrued revenue under alternative revenue programs									4.3	(16.9)
Total operating revenues									1,752.0	1,860.8
Spire | Gas Utility | Operating Segments | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,184.3	1,267.3
Spire | Gas Utility | Operating Segments | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues									383.0	433.9
Spire | Gas Utility | Operating Segments | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues									115.8	112.1
Spire | Gas Utility | Operating Segments | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues									38.4	41.9
Spire | Gas Utility | Operating Segments | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues									26.2	22.5
Spire | Gas Marketing | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues									87.9	83.7
Spire | Gas Marketing | Operating Segments | Revenue From Contracts With Retail Customers
Disaggregation Of Revenue [Line Items]
Total operating revenues									87.9	83.7
Spire | Other | Operating Segments
Disaggregation Of Revenue [Line Items]
Total operating revenues									$ 57.8	$ 21.5