Quarterly Financial Information-Unaudited (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Quarterly Financial Information-Unaudited
Revenue	$ 2,079,100	$ 2,067,500	$ 1,989,600	$ 2,017,300	$ 2,082,900	$ 2,095,200	$ 2,010,900	$ 2,029,200	$ 8,153,495	$ 8,218,180	$ 8,037,374
Cost of revenue	1,938,900	1,935,700	1,889,700	1,939,200	1,938,700	1,984,000	1,934,700	1,938,900	7,703,507	7,796,321	7,570,672
Gross profit	140,200	131,800	99,900	78,100	144,200	111,200	76,200	90,300	449,988	421,859	466,702
Equity in earnings of joint ventures	6,400	4,100	7,900	5,900	10,400	12,300	16,900	9,000	24,319	48,650	44,819
General and administrative expenses	(23,900)	(24,000)	(27,300)	(22,100)	(17,700)	(20,700)	(19,900)	(22,600)	(97,318)	(80,903)	(90,298)
Goodwill impairment					(336,000)					(336,000)
Income from operations	122,700	111,900	80,500	61,900	(199,100)	102,800	73,200	76,700	376,989	53,606	421,223
Other income	1,500	1,200	100	700	2,000	1,500	4,800	2,300	3,522	10,603	5,008
Interest expense	(10,200)	(11,700)	(11,900)	(10,900)	(11,000)	(13,100)	(11,600)	(11,000)	(44,737)	(46,726)	(42,051)
Income before income tax expense	114,000	101,400	68,700	51,700	(208,100)	91,200	66,400	68,000	335,774	17,483	384,180
Income tax expense	35,800	30,100	14,000	12,700	16,700	21,400	16,700	19,600	92,578	74,416	100,090
Net income (loss)	78,200	71,300	54,700	39,000	(224,800)	69,800	49,700	48,400	243,196	(56,933)	284,090
Noncontrolling interests in income of consolidated subsidiaries, net of tax	(1,700)	(500)	(900)	(900)	(100)	(400)	(700)	(500)	(3,953)	(1,634)	(8,290)
Net income (loss) attributable to AECOM	$ 76,500	$ 70,800	$ 53,800	$ 38,100	$ (224,900)	$ 69,400	$ 49,000	$ 47,900	$ 239,243	$ (58,567)	$ 275,800
Net income (loss) attributable to AECOM per share:
Basic (in dollars per share)	$ 0.78	$ 0.71	$ 0.54	$ 0.36	$ (2.05)	$ 0.63	$ 0.43	$ 0.42	$ 2.38	$ (0.52)	$ 2.35
Diluted (in dollars per share)	$ 0.77	$ 0.70	$ 0.53	$ 0.36	$ (2.05)	$ 0.63	$ 0.43	$ 0.42	$ 2.35	$ (0.52)	$ 2.33
Weighted average shares outstanding:
Basic (in shares)	98,000	99,300	100,400	104,800	110,000	110,200	113,400	114,000	100,618	111,875	117,396
Diluted (in shares)	99,700	100,800	101,800	105,500	110,000	110,800	114,300	114,600	101,942	111,875	118,345