Business Acquisitions, Goodwill and Intangible Assets (Details 2) (USD $)	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2013
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	$ 1,937,338,000us-gaap_Goodwill	$ 1,811,800,000us-gaap_Goodwill
Post-Acquisition Adjustments	3,846,900,000	79,100,000
Foreign Exchange Impact	9,700,000us-gaap_GoodwillTranslationAdjustments	5,000,000us-gaap_GoodwillTranslationAdjustments
Acquired	(128,500,000)us-gaap_GoodwillAcquiredDuringPeriod	(8,500,000)us-gaap_GoodwillAcquiredDuringPeriod
Goodwill at the end of the period	5,665,369,000us-gaap_Goodwill	1,887,400,000us-gaap_Goodwill
Design and Consulting Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	1,479,200,000	1,414,100,000
Post-Acquisition Adjustments	1,768,200,000	79,100,000
Foreign Exchange Impact	5,500,000	5,000,000
Acquired	(60,400,000)	(8,500,000)
Goodwill at the end of the period	3,192,500,000	1,489,700,000
Construction Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	276,900,000		216,500,000
Post-Acquisition Adjustments	370,200,000
Foreign Exchange Impact	4,200,000
Acquired	(24,100,000)
Goodwill at the end of the period	627,200,000	216,500,000	216,500,000
Management Services
Changes in the carrying value of goodwill by reporting segment
Goodwill at the beginning of the period	181,200,000		181,200,000
Post-Acquisition Adjustments	1,708,500,000
Acquired	(44,000,000)
Goodwill at the end of the period	$ 1,845,700,000	$ 181,200,000	$ 181,200,000

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

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-Section 45

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

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-Section 25

-Paragraph 16

-URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e961-128460

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