Reportable Segments and Geographic Information (Details) (USD $)	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 item	Sep. 30, 2012	Sep. 30, 2011
Reportable Segments and Geographic Information
Number of reportable segments									2
Summarized financial information concerning the Company's reportable segments
Revenue	$ 2,079,100,000	$ 2,067,500,000	$ 1,989,600,000	$ 2,017,300,000	$ 2,082,900,000	$ 2,095,200,000	$ 2,010,900,000	$ 2,029,200,000	$ 8,153,495,000	$ 8,218,180,000	$ 8,037,374,000
Revenue, net of other direct costs									4,977,000,000	5,183,900,000	5,180,800,000
Gross profit	140,200,000	131,800,000	99,900,000	78,100,000	144,200,000	111,200,000	76,200,000	90,300,000	449,988,000	421,859,000	466,702,000
Equity in earnings of joint ventures	6,400,000	4,100,000	7,900,000	5,900,000	10,400,000	12,300,000	16,900,000	9,000,000	24,319,000	48,650,000	44,819,000
General and administrative expenses	(23,900,000)	(24,000,000)	(27,300,000)	(22,100,000)	(17,700,000)	(20,700,000)	(19,900,000)	(22,600,000)	(97,318,000)	(80,903,000)	(90,298,000)
Goodwill impairment					(336,000,000)					(336,000,000)
Operating income (loss)	122,700,000	111,900,000	80,500,000	61,900,000	(199,100,000)	102,800,000	73,200,000	76,700,000	376,989,000	53,606,000	421,223,000
Segment assets	5,665,623,000				5,664,568,000				5,665,623,000	5,664,568,000	5,789,300,000
Gross profit as a % of revenue									5.50%	5.10%	5.80%
Gross profit as a % of revenue, net of other direct costs									9.00%	8.10%	9.00%
Professional Technical Services
Summarized financial information concerning the Company's reportable segments
Revenue									7,242,900,000	7,276,900,000	6,877,100,000
Revenue, net of other direct costs									4,416,400,000	4,607,300,000	4,612,200,000
Gross profit									416,900,000	423,800,000	417,700,000
Equity in earnings of joint ventures									12,300,000	16,800,000	15,300,000
Goodwill impairment										(155,000,000)
Operating income (loss)									429,200,000	285,600,000	433,000,000
Segment assets	5,761,100,000				5,557,200,000				5,761,100,000	5,557,200,000	5,296,700,000
Gross profit as a % of revenue									5.80%	5.80%	6.10%
Gross profit as a % of revenue, net of other direct costs									9.40%	9.20%	9.10%
Management Support Services
Summarized financial information concerning the Company's reportable segments
Revenue									910,600,000	941,300,000	1,160,300,000
Revenue, net of other direct costs									560,600,000	576,600,000	568,600,000
Gross profit									33,100,000	(1,900,000)	49,000,000
Equity in earnings of joint ventures									12,000,000	31,800,000	29,500,000
Goodwill impairment										(181,000,000)
Operating income (loss)									45,100,000	(151,100,000)	78,500,000
Segment assets	541,900,000				564,800,000				541,900,000	564,800,000	740,400,000
Gross profit as a % of revenue									3.60%	(0.20%)	4.20%
Gross profit as a % of revenue, net of other direct costs									5.90%	(0.30%)	8.60%
Corporate
Summarized financial information concerning the Company's reportable segments
General and administrative expenses									(97,300,000)	(80,900,000)	(90,300,000)
Operating income (loss)									(97,300,000)	(80,900,000)	(90,300,000)
Segment assets	$ (637,400,000)				$ (457,400,000)				$ (637,400,000)	$ (457,400,000)	$ (247,800,000)