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Commitments and Contingencies (Details) - USD ($) $ in Millions			1 Months Ended		6 Months Ended	72 Months Ended
	Jan. 31, 2020	Dec. 06, 2019	Dec. 31, 2019	Dec. 31, 2014	Mar. 31, 2020	Sep. 30, 2019	Apr. 30, 2019	Mar. 31, 2019
Commitments and Contingencies
Percentage of claim recoveries due to seller	10.00%
Percentage of claim recoveries due to purchaser	90.00%
Department of Energy Deactivation, Demolition, And Removal Project | Department of Energy | Maximum
Commitments and Contingencies
Cost-reimbursement at risk					$ 106.0
Department of Energy Deactivation, Demolition, And Removal Project | AECOM E&C and DOE | Minimum
Commitments and Contingencies
Cost-reimbursement at risk					106.0
Department of Energy Deactivation, Demolition, And Removal Project | AECOM E&C and DOE | Maximum
Commitments and Contingencies
Cost-reimbursement at risk					146.0
Department of Energy Deactivation, Demolition, And Removal Project | AECOM E&C
Commitments and Contingencies
Claim to DOE, including additional fees		$ 60.4		$ 103.0
Department of Energy Deactivation, Demolition, And Removal Project | AECOM E&C | Minimum
Commitments and Contingencies
Cost-reimbursement at risk					146.0
Claim to DOE, including additional fees						$ 148.5
Department of Energy Deactivation, Demolition, And Removal Project | AECOM E&C | Maximum
Commitments and Contingencies
Claim to DOE, including additional fees						$ 329.4
Refinery Turnaround Project
Commitments and Contingencies
Damages paid, value			$ 93.0
Refinery Turnaround Project | AECOM E&C
Commitments and Contingencies
Damages claimed, value					144.0
Recoverable from refinery owner					90.0
Loss contingency, receivable amount							$ 132.0	$ 79.0
AECOM-Canyon Equity Fund, L.P
Commitments and Contingencies
Capital commitments					$ 35.0
Term of capital commitments					10 years
Standby letter of credit
Commitments and Contingencies
Contingency liability					$ 471.2
Surety Bond
Commitments and Contingencies
Surety bonds issued					$ 4,800.0

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