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Income Taxes - Gross Unrecognized (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits |  |
Balance at the beginning of the year | $ 47.1 | $ 55.7
Gross increase in current period's tax positions | 4.3 | 2.8
Gross increase in prior years' tax positions | 7.5 |
Gross decrease in prior years' tax positions |  | (7.9)
Decrease due to settlement with tax authorities | (1.3) | (0.5)
Decrease due to lapse of statute of limitations |  | (3.5)
Gross change due to foreign exchange fluctuations | (11.2) | 0.5
Balance at the end of the year | 46.4 | 47.1
Income Taxes, additional disclosures |  |
Accrued interest, excluding any related income tax benefits | 20.0 | 18.9
Accrued penalties, excluding any related income tax benefits | $ 3.9 | $ 2.7

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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 10B -URI http://asc.fasb.org/extlink&oid=120406818&loc=SL37586934-109318 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of statutory penalties accrued for a tax position claimed or expected to be claimed by the entity, in its tax return. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319 + Details
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- Definition Amount of interest expense accrued for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319 + Details
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