Segment Information - Schedule of Total Services Revenues for Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Apr. 01, 2016	Mar. 31, 2017	Apr. 01, 2016
Segment Reporting Information [Line Items]
Revenues	$ 2,302,567	$ 2,781,763	$ 4,854,171	$ 5,629,697
Project Services
Segment Reporting Information [Line Items]
Revenues	1,303,800	1,534,000	2,624,400	2,930,300
Process, Scientific, and Systems Consulting
Segment Reporting Information [Line Items]
Revenues	194,400	156,500	401,200	415,400
Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	1,498,200	1,690,500	3,025,600	3,345,700
Construction
Segment Reporting Information [Line Items]
Revenues	594,600	807,200	1,404,500	1,695,600
Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	209,700	284,100	424,100	588,400
Field Services
Segment Reporting Information [Line Items]
Revenues	804,300	1,091,300	1,828,600	2,284,000
Aerospace & Technology
Segment Reporting Information [Line Items]
Revenues	577,040	669,464	1,154,476	1,339,655
Aerospace & Technology | Project Services
Segment Reporting Information [Line Items]
Revenues	265,100	264,200	482,500	466,800
Aerospace & Technology | Process, Scientific, and Systems Consulting
Segment Reporting Information [Line Items]
Revenues	184,900	140,000	384,800	382,900
Aerospace & Technology | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	450,000	404,200	867,300	849,700
Aerospace & Technology | Construction
Segment Reporting Information [Line Items]
Revenues	10,300	79,400	54,800	105,800
Aerospace & Technology | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	116,700	185,900	232,400	384,100
Aerospace & Technology | Field Services
Segment Reporting Information [Line Items]
Revenues	127,000	265,300	287,200	489,900
Buildings & Infrastructure
Segment Reporting Information [Line Items]
Revenues	585,242	579,128	1,165,859	1,142,458
Buildings & Infrastructure | Project Services
Segment Reporting Information [Line Items]
Revenues	532,000	542,600	1,046,000	1,066,800
Buildings & Infrastructure | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	532,000	542,600	1,046,000	1,066,800
Buildings & Infrastructure | Construction
Segment Reporting Information [Line Items]
Revenues	49,600	24,400	112,500	50,900
Buildings & Infrastructure | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	3,600	12,200	7,400	24,800
Buildings & Infrastructure | Field Services
Segment Reporting Information [Line Items]
Revenues	53,200	36,600	119,900	75,700
Industrial
Segment Reporting Information [Line Items]
Revenues	582,458	666,556	1,334,196	1,338,656
Industrial | Project Services
Segment Reporting Information [Line Items]
Revenues	166,900	218,100	383,300	424,300
Industrial | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	166,900	218,100	383,300	424,300
Industrial | Construction
Segment Reporting Information [Line Items]
Revenues	326,200	363,000	766,700	736,000
Industrial | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	89,400	85,400	184,200	178,400
Industrial | Field Services
Segment Reporting Information [Line Items]
Revenues	415,600	448,400	950,900	914,400
Petroleum & Chemicals
Segment Reporting Information [Line Items]
Revenues	557,827	866,615	1,199,640	1,808,928
Petroleum & Chemicals | Project Services
Segment Reporting Information [Line Items]
Revenues	339,800	509,100	712,600	972,400
Petroleum & Chemicals | Process, Scientific, and Systems Consulting
Segment Reporting Information [Line Items]
Revenues	9,500	16,500	16,400	32,500
Petroleum & Chemicals | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	349,300	525,700	729,000	1,004,900
Petroleum & Chemicals | Construction
Segment Reporting Information [Line Items]
Revenues	208,500	340,400	470,500	802,900
Petroleum & Chemicals | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	0	600	100	1,200
Petroleum & Chemicals | Field Services
Segment Reporting Information [Line Items]
Revenues	$ 208,500	$ 341,000	$ 470,600	$ 804,000