Revenue - Schedule of Operating Revenue Information (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
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,191.3	1,304.7
Changes in accrued revenue under alternative revenue programs									2.3	(12.9)
Total operating revenues	158.2	203.9	457.5	374.0	130.6	191.4	556.6	413.2	1,193.6	1,291.8	1,285.6
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues									859.6	945.9
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues									241.4	283.8
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues									32.9	33.1
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues									35.1	41.9
Spire Missouri | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues									22.3
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues									455.2	469.0
Changes in accrued revenue under alternative revenue programs									(0.2)	(3.5)
Total operating revenues	$ 62.1	$ 81.2	$ 185.5	$ 126.2	$ 60.8	$ 90.8	$ 180.4	$ 133.5	455.0	465.5	$ 500.7
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues									267.8	265.3
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues									109.4	113.5
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues									72.9	69.5
Spire Alabama | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues									3.2
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues									$ 1.9	$ 20.7