Financing Arrangements and Long-term Debt - Schedule of Spire Missouri and Alabama Short-term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Spire Missouri | Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 331.1
Weighted average interest rate	2.20%
Minimum borrowing outstanding	$ 239.4
Maximum borrowing outstanding	429.5
Borrowings outstanding	$ 288.1	$ 386.4	$ 308.0
Weighted average interest rate	2.10%	2.30%	3.10%
Spire Alabama | Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 115.0
Weighted average interest rate	2.20%
Minimum borrowing outstanding	$ 43.8
Maximum borrowing outstanding	161.3
Borrowings outstanding	$ 67.1	$ 128.7	$ 164.2
Weighted average interest rate	2.10%	2.30%	3.10%
Spire | Commercial Paper Notes
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 727.7
Weighted average interest rate	2.20%
Minimum borrowing outstanding	$ 510.9
Maximum borrowing outstanding	856.6
Borrowings outstanding	$ 518.9	$ 743.2	$ 626.1
Weighted average interest rate	2.10%	2.30%	3.10%