Segment Information - Schedule of Total Services Revenues for Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jul. 01, 2016	Jun. 30, 2017	Jul. 01, 2016
Segment Reporting Information [Line Items]
Revenues	$ 2,514,751	$ 2,693,873	$ 7,368,922	$ 8,323,570
Project Services
Segment Reporting Information [Line Items]
Revenues	1,447,362	1,226,234	4,071,762	4,120,901
Process, Scientific, and Systems Consulting
Segment Reporting Information [Line Items]
Revenues	132,466	387,289	533,666	838,358
Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	1,579,828	1,613,523	4,605,428	4,959,259
Construction
Segment Reporting Information [Line Items]
Revenues	712,687	708,880	2,117,160	2,404,452
Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	222,236	371,470	646,334	959,859
Field Services
Segment Reporting Information [Line Items]
Revenues	934,923	1,080,350	2,763,494	3,364,311
Aerospace & Technology
Segment Reporting Information [Line Items]
Revenues	585,432	667,785	1,739,908	2,007,440
Aerospace & Technology | Project Services
Segment Reporting Information [Line Items]
Revenues	317,343	35,311	799,842	502,105
Aerospace & Technology | Process, Scientific, and Systems Consulting
Segment Reporting Information [Line Items]
Revenues	121,450	371,954	506,250	790,533
Aerospace & Technology | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	438,793	407,265	1,306,092	1,292,638
Aerospace & Technology | Construction
Segment Reporting Information [Line Items]
Revenues	23,228	7,278	78,005	77,464
Aerospace & Technology | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	123,411	253,242	355,811	637,338
Aerospace & Technology | Field Services
Segment Reporting Information [Line Items]
Revenues	146,639	260,520	433,816	714,802
Buildings & Infrastructure
Segment Reporting Information [Line Items]
Revenues	647,252	553,546	1,813,111	1,696,004
Buildings & Infrastructure | Project Services
Segment Reporting Information [Line Items]
Revenues	587,543	503,336	1,633,543	1,570,143
Buildings & Infrastructure | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	587,543	503,336	1,633,543	1,570,143
Buildings & Infrastructure | Construction
Segment Reporting Information [Line Items]
Revenues	56,067	37,518	168,526	88,383
Buildings & Infrastructure | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	3,642	12,692	11,042	37,478
Buildings & Infrastructure | Field Services
Segment Reporting Information [Line Items]
Revenues	59,709	50,210	179,568	125,861
Industrial
Segment Reporting Information [Line Items]
Revenues	681,588	705,996	2,015,784	2,044,652
Industrial | Project Services
Segment Reporting Information [Line Items]
Revenues	234,670	222,263	617,970	646,543
Industrial | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	234,670	222,263	617,970	646,543
Industrial | Construction
Segment Reporting Information [Line Items]
Revenues	352,002	378,289	1,118,697	1,114,312
Industrial | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	94,916	105,444	279,117	283,797
Industrial | Field Services
Segment Reporting Information [Line Items]
Revenues	446,918	483,733	1,397,814	1,398,109
Petroleum & Chemicals
Segment Reporting Information [Line Items]
Revenues	600,479	766,546	1,800,119	2,575,474
Petroleum & Chemicals | Project Services
Segment Reporting Information [Line Items]
Revenues	307,806	465,324	1,020,407	1,402,110
Petroleum & Chemicals | Process, Scientific, and Systems Consulting
Segment Reporting Information [Line Items]
Revenues	11,016	15,335	27,416	47,825
Petroleum & Chemicals | Technical Professional Services
Segment Reporting Information [Line Items]
Revenues	318,822	480,659	1,047,823	1,449,935
Petroleum & Chemicals | Construction
Segment Reporting Information [Line Items]
Revenues	281,390	285,795	751,932	1,124,293
Petroleum & Chemicals | Operations and Maintenance ("O&M")
Segment Reporting Information [Line Items]
Revenues	267	92	364	1,246
Petroleum & Chemicals | Field Services
Segment Reporting Information [Line Items]
Revenues	$ 281,657	$ 285,887	$ 752,296	$ 1,125,539