Summary of Significant Accounting Policies - Summary of 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, 2025	Dec. 31, 2024	Sep. 30, 2025
Accounting Policies [Line Items]
Accrued capital expenditures	$ 51.0	$ 73.7	$ 82.5
Spire Missouri [Member]
Accounting Policies [Line Items]
Accrued capital expenditures	39.7	38.1	61.8
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Accrued capital expenditures	$ 3.8	$ 10.5	$ 9.5