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Business Acquisitions, Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2018 USD ($)
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	$ 5,992,881
Measurement period adjustments	91,000
Impairment	(125,400)
Foreign Exchange Impact	(30,500)
Goodwill at the end of the period	5,928,029
Design and Consulting Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	3,218,900
Foreign Exchange Impact	(24,100)
Goodwill at the end of the period	3,194,800
Construction Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	1,049,900
Measurement period adjustments	91,000
Impairment	(125,400)
Foreign Exchange Impact	(8,700)
Goodwill at the end of the period	1,006,800
Management Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	1,724,100
Foreign Exchange Impact	2,300
Goodwill at the end of the period	$ 1,726,400

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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- Definition

The amount of an adjustment to a previously reported goodwill impairment charge, which will be recognized in the subsequent reporting period.

+ References

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Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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