Income Taxes - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Spire
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 8.1	$ 11.0	$ 10.0
Decrease related to tax law changes	0.0	(4.0)	0.0
Increases related to tax positions taken in current year	4.5	1.2	2.4
Reductions related to tax positions taken in prior year	(1.9)	0.0	0.0
Reductions due to lapse of applicable statute of limitations	0.0	(0.1)	(1.4)
Unrecognized tax benefits, end of year	10.7	8.1	11.0
Spire Missouri
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	7.8	10.7	9.7
Decrease related to tax law changes	0.0	(4.0)	0.0
Increases related to tax positions taken in current year	4.5	1.1	2.4
Reductions related to tax positions taken in prior year	(1.9)	0.0	0.0
Reductions due to lapse of applicable statute of limitations	0.0	0.0	(1.4)
Unrecognized tax benefits, end of year	$ 10.4	$ 7.8	$ 10.7

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Amount of unrecognized tax benefits.

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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