Summary of Significant Accounting Policies - Accrued Capital Expenditures (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 45.8	$ 46.2	$ 80.6
Spire Missouri
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	20.2	18.1	40.1
Spire Alabama
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items]
Accruals for capital expenditures	$ 9.8	$ 9.7	$ 11.9