Income Taxes - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Spire
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 10.7	$ 8.1	$ 11.0
Decrease related to tax law changes	0.0	0.0	(4.0)
Increases related to tax positions taken in current year	2.6	4.5	1.2
Reductions related to tax positions taken in prior year	0.0	(1.9)	0.0
Reductions due to lapse of applicable statute of limitations	(0.1)	0.0	(0.1)
Unrecognized tax benefits, end of year	13.2	10.7	8.1
Spire Missouri
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	10.4	7.8	10.7
Decrease related to tax law changes	0.0	0.0	(4.0)
Increases related to tax positions taken in current year	2.6	4.5	1.1
Reductions related to tax positions taken in prior year	0.0	(1.9)	0.0
Reductions due to lapse of applicable statute of limitations	0.0	0.0	0.0
Unrecognized tax benefits, end of year	$ 13.0	$ 10.4	$ 7.8