Summary of Significant Accounting Policies - Accrued Capital Expenditures (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 51.4	$ 63.6	$ 80.6
Spire Missouri
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	21.5	19.0	40.1
Spire Alabama
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 17.6	$ 12.1	$ 11.9