INCOME TAXES - Unrecognized Tax Benefit Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 11.0	$ 10.0	$ 7.1
Decrease related to tax law changes	(4.0)	0.0	0.0
Increases related to tax positions taken in current year	1.2	2.4	3.4
Reductions due to lapse of applicable statute of limitations	(0.1)	(1.4)	(0.5)
Unrecognized tax benefits, end of year	8.1	11.0	10.0
Spire Missouri
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning of year	10.7	9.7	6.9
Decrease related to tax law changes	(4.0)	0.0	0.0
Increases related to tax positions taken in current year	1.1	2.4	3.3
Reductions due to lapse of applicable statute of limitations	0.0	(1.4)	(0.5)
Unrecognized tax benefits, end of year	$ 7.8	$ 10.7	$ 9.7

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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- Definition

Amount of unrecognized tax benefits.

+ References

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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