Income Taxes - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Spire
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 13.2	$ 10.7	$ 8.1
Increases related to tax positions taken in current year	3.2	2.6	4.5
Reductions related to tax positions taken in prior year	0.0	0.0	(1.9)
Reductions due to lapse of applicable statute of limitations	0.0	(0.1)	0.0
Unrecognized tax benefits, end of year	16.4	13.2	10.7
Spire Missouri
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	13.0	10.4	7.8
Increases related to tax positions taken in current year	3.1	2.6	4.5
Reductions related to tax positions taken in prior year	0.0	0.0	(1.9)
Reductions due to lapse of applicable statute of limitations	0.0	0.0	0.0
Unrecognized tax benefits, end of year	$ 16.1	$ 13.0	$ 10.4