Summary of Significant Accounting Policies - Accrued Capital Expenditures (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 67.6	$ 80.6	$ 62.1
Spire Missouri
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	34.3	40.1	36.7
Spire Alabama
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 17.0	$ 11.9	$ 8.9