Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 1,104.9	$ 715.5	$ 1,617.5	$ 1,282.4
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	790.3	450.2	1,142.5	825.8
Changes in accrued revenue under alternative revenue programs	0.6	7.3	3.9	5.7
Total operating revenues	790.9	457.5	1,146.4	831.5
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	365.4	331.1	628.4	607.0
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	304.0	99.2	373.9	180.0
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.9	9.7	18.9	18.7
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	128.5	9.3	135.9	18.7
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	(17.5)	0.9	(14.6)	1.4
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	225.5	188.3	339.6	315.1
Changes in accrued revenue under alternative revenue programs	(2.9)	(2.8)	(3.4)	(3.4)
Total operating revenues	222.6	185.5	336.2	311.7
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	144.8	120.5	214.0	194.8
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	53.2	46.2	77.5	77.5
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	21.7	19.9	42.2	39.5
Spire Alabama | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	1.9		3.9
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 3.9	$ 1.7	$ 2.0	$ 3.3