Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 715.5	$ 803.5	$ 1,282.4	$ 1,405.5
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues	450.2	565.7	825.8	983.4
Changes in accrued revenue under alternative revenue programs	7.3	(9.1)	5.7	(13.6)
Total Operating Revenues	457.5	556.6	831.5	969.8
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	331.1	410.9	607.0	713.0
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	99.2	129.2	180.0	218.3
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.7	9.9	18.7	18.9
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues	9.3	11.5	18.7	26.4
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	0.9	4.2	1.4	6.8
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues	188.3	185.1	315.1	319.7
Changes in accrued revenue under alternative revenue programs	(2.8)	(4.7)	(3.4)	(5.8)
Total Operating Revenues	185.5	180.4	311.7	313.9
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues	120.5	116.9	194.8	192.3
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues	46.2	44.1	77.5	75.5
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues	19.9	18.8	39.5	37.0
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues	$ 1.7	$ 5.3	$ 3.3	$ 14.9