Summary of Significant Accounting Policies - Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Sep. 30, 2024
Accounting Policies [Line Items]
Accrued capital expenditures	$ 73.7	$ 90.5	$ 116.5
Spire Missouri [Member]
Accounting Policies [Line Items]
Accrued capital expenditures	38.1	38.1	67.4
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Accrued capital expenditures	$ 10.5	$ 2.4	$ 14.1