Joint Ventures and Variable Interest Entities (Details) (USD $)	3 Months Ended		9 Months Ended
	Jun. 30, 2014 item	Jun. 30, 2013	Jun. 30, 2014 item	Jun. 30, 2013	Sep. 30, 2013
Financial information
Number of employees in some joint ventures	0		0
Current assets	$ 3,121,810,000		$ 3,121,810,000		$ 3,131,602,000
TOTAL ASSETS	5,753,742,000		5,753,742,000		5,665,623,000
Current liabilities	2,053,860,000		2,053,860,000		2,053,549,000
TOTAL LIABILITIES	3,470,269,000		3,470,269,000		3,591,529,000
Total AECOM equity	2,197,512,000		2,197,512,000		2,021,443,000
Noncontrolling interests	85,961,000		85,961,000		52,651,000
TOTAL STOCKHOLDERS' EQUITY	2,283,473,000		2,283,473,000		2,074,094,000
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	5,753,742,000		5,753,742,000		5,665,623,000
AECOM's investment in joint ventures	133,766,000		133,766,000		106,422,000
Joint ventures summarized financial information
Total revenue	1,968,155,000	2,067,490,000	5,794,254,000	6,074,408,000
Gross profit	108,540,000	131,814,000	274,145,000	309,775,000
Summary of AECOM's equity in earnings of unconsolidated joint ventures:
Total	5,896,000	4,094,000	49,415,000	17,855,000
Greater than or equal to
Financial information
Number of partners required to form joint ventures	2		2
Consolidated joint ventures
Financial information
Current assets	236,200,000		236,200,000		185,700,000
Non-current assets	109,300,000		109,300,000
TOTAL ASSETS	345,500,000		345,500,000		185,700,000
Current liabilities	172,100,000		172,100,000		38,900,000
TOTAL LIABILITIES	172,100,000		172,100,000		38,900,000
Total AECOM equity	99,500,000		99,500,000		106,800,000
Noncontrolling interests	73,900,000		73,900,000		40,000,000
TOTAL STOCKHOLDERS' EQUITY	173,400,000		173,400,000		146,800,000
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	345,500,000		345,500,000		185,700,000
Joint ventures summarized financial information
Total revenue			380,200,000	392,600,000
Consolidated joint venture during the quarter ended 06/30/2014
Financial information
TOTAL ASSETS	207,800,000		207,800,000
TOTAL LIABILITIES	48,100,000		48,100,000
Equity in earnings of unconsolidated joint ventures
Gain (loss) from equity in earnings of joint ventures due to the change in control			37,400,000
Gain (loss) from equity in earnings of joint ventures due to the change in control, net of tax			23,400,000
Fair value of the previously held equity interest, excluding control premium	58,000,000		58,000,000
Unconsolidated joint ventures
Financial information
Current assets	435,800,000		435,800,000		525,500,000
Non-current assets	244,800,000		244,800,000		98,700,000
TOTAL ASSETS	680,600,000		680,600,000		624,200,000
Current liabilities	337,800,000		337,800,000		384,100,000
Non-current liabilities	48,300,000		48,300,000		17,500,000
TOTAL LIABILITIES	386,100,000		386,100,000		401,600,000
TOTAL STOCKHOLDERS' EQUITY	294,500,000		294,500,000		222,600,000
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	680,600,000		680,600,000		624,200,000
AECOM's investment in joint ventures	133,800,000		133,800,000		106,400,000
Joint ventures summarized financial information
Total revenue			1,455,300,000	1,504,600,000
Gross profit			39,100,000	49,500,000
Summary of AECOM's equity in earnings of unconsolidated joint ventures:
Pass through joint ventures			5,500,000	4,800,000
Other joint ventures			44,000,000	13,100,000
Total			$ 49,500,000	$ 17,900,000