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Commitments and contingencies - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract] |  |  |
Performance bond commitments | $ 36.8 |  |
Operating lease expense | 101.7 | $ 77.0 | $ 80.6
Operating sublease income | 0.9 |  |
Sublease income, payments to be received, 2020 | $ 0.5 |  |
Percentage of employees covered by collective bargaining agreements | 8.00% |  |
Loss contingency, maximum potential loss | $ 31.0 |  |

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- Definition The percentage of employees covered by collective bargaining agreements as of the balance sheet date when it serves as a benchmark in a concentration of risk calculation. + References No definition available. + Details
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- Definition The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248 + Details
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- Definition Amount of lease payments to be received by lessor in next fiscal year following latest fiscal year for operating lease. Excludes interim and annual periods when interim periods are reported on rolling approach, from latest statement of financial position date. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 30 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=77888428&loc=SL77919396-209981 + Details
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- Definition The amount of revenue recognized for the period under subleasing arrangements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 55 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=77891322&loc=d3e41620-112719 + Details
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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
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