Summary of Significant Accounting Policies - Accrued Capital Expenditures (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 60.2	$ 33.8	$ 62.1
Spire Missouri
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	26.3	21.9	36.7
Spire Alabama
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 9.0	$ 9.9	$ 8.9