Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Sep. 30, 2025
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Company records amounts representing its probable estimated liabilities relating to claims, guarantees, litigation, audits and investigations. The Company relies in part on qualified actuaries to assist it in determining the level of reserves to establish for insurance-related claims that are known and have been asserted against it, and for insurance-related claims that are believed to have been incurred based on actuarial analysis, but have not yet been reported to the Company’s claims administrators as of the respective balance sheet dates. The Company includes any adjustments to such insurance reserves in its consolidated results of operations. The Company’s reasonably possible loss disclosures are presented on a gross basis prior to the consideration of insurance recoveries. The Company does not record gain contingencies until they are realized. In the ordinary course of business, the Company may not be aware that it or its affiliates are under investigation and may not be aware of whether or not a known investigation has been concluded. In the ordinary course of business, the Company may enter into various arrangements providing financial or performance assurance to clients, lenders, or partners. Such arrangements include standby letters of credit, surety bonds, and corporate guarantees to support the creditworthiness or the project execution commitments of its affiliates, partnerships and joint ventures. The Company’s unsecured credit arrangements are used for standby letters of credit issued in connection with general and professional liability insurance programs and for contract performance guarantees. At September 30, 2025 and 2024, these outstanding standby letters of credit totaled $899.4 million and $934.5 million, respectively. As of September 30, 2025, the Company had $367.4 available under these unsecured credit facilities. Performance arrangements typically have various expiration dates ranging from the completion of the project contract and extending beyond contract completion in some circumstances such as for warranties. The Company may also guarantee that a project, when complete, will achieve specified performance standards. If the project subsequently fails to meet guaranteed performance standards, the Company may incur additional costs, pay liquidated damages or be held responsible for the costs incurred by the client to achieve the required performance standards. The potential payment amount of an outstanding performance arrangement is typically the remaining cost of work to be performed by or on behalf of third parties. Generally, under joint venture arrangements, if a partner is financially unable to complete its share of the contract, the other partner(s) may be required to complete those activities. At September 30, 2025, the Company was contingently liable in the amount of approximately $903.8 million in issued standby letters of credit and $5.6 billion in issued surety bonds primarily to support project execution. In the ordinary course of business, the Company enters into various agreements providing financial or performance assurances to clients on behalf of certain unconsolidated partnerships, joint ventures and other jointly executed contracts. These agreements are entered into primarily to support the project execution commitments of these entities. The Company’s investment adviser jointly manages and sponsors the AECOM-Canyon Equity Fund, L.P. (the “Fund”), in which the Company indirectly holds an equity interest and has an ongoing capital commitment to fund investments. At September 30, 2025, the Company has capital commitments of $5.1 million to the Fund over the next three years In addition, in connection with the investment activities of AECOM Capital, the Company provides guarantees of certain contractual obligations, including guarantees for completion of projects, limited debt repayment, environmental indemnity obligations and other lender required guarantees. In February 2024, the Company was informed of a potential liability as one of the indemnitors on a divested business’ surety bonds. The Company does not have sufficient information to determine the range of potential impacts; however, it is reasonably possible that the Company may incur additional costs related to these bonds. In connection with the resolution of contingencies related to the sale of the civil infrastructure construction business, the Company agreed to act as an additional guarantor on the counterparty’s existing debt, which was extended to March 2028.