Selected Quarterly Information - Unaudited (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2016	Jul. 01, 2016	Apr. 01, 2016	Jan. 01, 2016	Oct. 02, 2015	Jun. 26, 2015	Mar. 27, 2015	Dec. 26, 2014	Sep. 26, 2014	Jun. 27, 2014	Mar. 28, 2014	Dec. 27, 2013	Sep. 30, 2016	Oct. 02, 2015	Sep. 26, 2014
Finite-Lived Intangible Assets [Line Items]
Revenues	$ 2,640,587	$ 2,693,873	$ 2,781,763	$ 2,847,934	$ 3,116,954	$ 2,907,541	$ 2,903,332	$ 3,187,005	$ 3,218,442	$ 3,231,791	$ 3,176,033	$ 3,068,891	$ 10,964,157	$ 12,114,832	$ 12,695,157
Operating profit	82,811	109,556	86,781	59,450	53,825	100,434	133,045	158,223	136,650	123,937	122,434	145,047	338,598	445,527	528,068
Earnings before taxes	35,673	102,807	90,456	57,787	49,292	97,188	128,962	154,695	144,805	118,046	132,394	146,921	286,723	430,137	542,166
Net Earnings of the Group	29,883	70,937	63,389	50,306	37,269	97,308	88,110	106,195	91,054	71,309	90,800	98,949	214,515	328,882	352,112
Net earnings attributable to Jacobs	$ 29,644	$ 69,055	$ 65,250	$ 46,514	$ 29,863	$ 91,062	$ 81,967	$ 100,079	$ 86,074	$ 64,842	$ 83,460	$ 93,732	$ 210,463	$ 302,971	$ 328,108
Earnings per share:
Basic (in dollars per share)	$ 0.25	$ 0.58	$ 0.54	$ 0.38	$ 0.00	$ 0.74	$ 0.65	$ 0.78	$ 0.66	$ 0.50	$ 0.64	$ 0.72	$ 1.75	$ 2.42	$ 2.51
Diluted (in dollars per share)	$ 0.24	$ 0.57	$ 0.54	$ 0.38	$ 0.24	$ 0.73	$ 0.64	$ 0.77	$ 0.65	$ 0.49	0.63	$ 0.71	$ 1.73	$ 2.40	$ 2.48
Restructuring Charges	$ 36,000	$ 25,800	$ 25,700	$ 48,100	$ 68,200	$ 30,100	$ 9,600		$ 30,400	$ 47,000			$ 187,907	$ 157,200	$ 93,300
Effect of restructuring charge on earnings per diluted share (in dollars per share)	$ (0.3)	$ (0.21)	$ (0.21)	$ (0.39)	$ (0.56)	$ (0.24)	$ (0.08)		$ (0.23)	$ (0.35)
Gain on sale of intellectual property															$ 12,147
Intellectual Property
Earnings per share:
Gain after tax on sale of intellectual property per diluted share (in dollars per share)											$ 0.05
Gain on sale of intellectual property											$ 6,400
Loss on Sale of Subsidiary [Member]
Earnings per share:
Restructuring Charges	$ 17,100
Effect of restructuring charge on earnings per diluted share (in dollars per share)	$ 0.14
Non-Cash Write-Off on Equity Investment [Member]
Earnings per share:
Restructuring Charges	$ 10,400
Effect of restructuring charge on earnings per diluted share (in dollars per share)	$ 0.09