Business Acquisitions, Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	$ 5,992,881	$ 5,823,800
Measurement period adjustments	91,000
Impairment	(125,400)
Acquisitions		127,100
Disposed		(1,800)
Foreign Exchange Impact	(37,400)	43,800
Goodwill at the end of the period	5,921,116	5,992,881
Design and Consulting Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	3,218,900	3,198,200
Acquisitions		3,800
Disposed		(1,800)
Foreign Exchange Impact	(29,700)	18,700
Goodwill at the end of the period	3,189,200	3,218,900
Construction Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	1,049,900	915,200
Measurement period adjustments	91,000
Impairment	(125,400)
Acquisitions		123,300
Foreign Exchange Impact	(6,600)	11,400
Goodwill at the end of the period	1,008,900	1,049,900
Management Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	1,724,100	1,710,400
Foreign Exchange Impact	(1,100)	13,700
Goodwill at the end of the period	$ 1,723,000	$ 1,724,100

Represents the amount of goodwill, disposed during period.

No definition available.

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- 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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- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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