Revenue - Schedule of Operating Revenue Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Disaggregation Of Revenue [Line Items]
Total Operating Revenues	$ 225.6	$ 321.3	$ 803.5	$ 602.0	$ 239.2	$ 350.6	$ 813.4	$ 561.8	$ 1,952.4	$ 1,965.0	$ 1,740.7
Spire Missouri
Disaggregation Of Revenue [Line Items]
Total operating revenues									1,304.7
Changes in accrued revenue under alternative revenue programs									(12.9)
Total Operating Revenues	130.6	191.4	556.6	413.2	144.6	215.5	533.2	392.3	1,291.8	1,285.6	1,171.9
Spire Missouri | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues									945.9
Spire Missouri | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues									283.8
Spire Missouri | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues									33.1
Spire Missouri | Off-system & Other Incentive
Disaggregation Of Revenue [Line Items]
Total operating revenues									41.9
Spire Alabama
Disaggregation Of Revenue [Line Items]
Total operating revenues									469.0
Changes in accrued revenue under alternative revenue programs									(3.5)
Total Operating Revenues	$ 60.8	$ 90.8	$ 180.4	$ 133.5	$ 61.3	$ 100.3	$ 218.3	$ 120.8	465.5	$ 500.7	$ 400.5
Spire Alabama | Residential
Disaggregation Of Revenue [Line Items]
Total operating revenues									265.3
Spire Alabama | Commercial & Industrial
Disaggregation Of Revenue [Line Items]
Total operating revenues									113.5
Spire Alabama | Transportation
Disaggregation Of Revenue [Line Items]
Total operating revenues									69.5
Spire Alabama | Other Customer Revenue
Disaggregation Of Revenue [Line Items]
Total operating revenues									$ 20.7