Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Activity within the reserve for unrecognized tax benefits
Balance at the beginning of the period	$ 12.1	$ 9.9	$ 7.7
Additions for tax positions related to the current year	0.3	0.3	0.7
Additions for tax positions of prior years	4.0	2.8	3.4
Reductions for tax positions of prior years	(0.4)	(0.6)	(1.5)
Settlements	(6.3)	(0.3)	(0.4)
Balance at the end of the period	9.7	12.1	$ 9.9
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	9.7	12.1
Additional amounts recognized (released) for interest and penalties	0.4	0.9
Other amounts accrued for interest and penalties	$ 0.7	2.0
Amount by which the unrecognized tax benefits could increase or decrease over the next 12 months		$ 2.5

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

The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date.

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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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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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