Earnings Per Share and Certain Related Information (Details)		3 Months Ended								12 Months Ended
	Jan. 17, 2019 USD ($) $ / shares shares	Oct. 02, 2020 USD ($) $ / shares	Jun. 26, 2020 USD ($) $ / shares	Mar. 27, 2020 USD ($) $ / shares	Dec. 27, 2019 USD ($) $ / shares	Sep. 27, 2019 USD ($) $ / shares	Jun. 28, 2019 USD ($) $ / shares	Mar. 29, 2019 USD ($) $ / shares	Dec. 28, 2018 USD ($) $ / shares	Oct. 02, 2020 USD ($) $ / shares shares	Sep. 27, 2019 USD ($) financial_institution $ / shares shares	Sep. 28, 2018 USD ($) $ / shares shares	Dec. 04, 2019 USD ($)	Jun. 05, 2019 USD ($)
Numerator for Basic and Diluted EPS:
Net earnings (loss) attributable to Jacobs from continuing operations		$ 69,519,000	$ 226,886,000	$ (121,967,000)	$ 179,423,000	$ 21,946,000	$ 89,365,000	$ 114,755,000	$ 64,894,000	$ 353,861,000	$ 290,960,000	$ (4,185,000)
Net earnings from continuing operations allocated to participating securities										(72,000)	(415,000)	0
Net earnings (loss) from continuing operations allocated to common stock for EPS calculation										353,789,000	290,545,000	(4,185,000)
Net earnings attributable to Jacobs from discontinued operations										137,984,000	557,019,000	167,616,000
Net earnings from discontinued operations allocated to participating securities										(28,000)	(795,000)	(808,000)
Net earnings from discontinued operations allocated to common stock for EPS calculation										137,956,000	556,224,000	166,808,000
Net earnings allocated to common stock for EPS calculation										$ 491,745,000	$ 846,769,000	$ 162,623,000
Denominator for Basic and Diluted EPS:
Weighted average basic shares (in shares) | shares										131,541,000	138,104,000	138,182,000
Shares allocated to participating securities (in shares) | shares										(27,000)	(197,000)	(646,000)
Shares used for calculating basic EPS attributable to common stock (in shares) | shares										131,514,000	137,907,000	137,536,000
Effect of dilutive securities:
Stock compensation plans (in shares) | shares										1,207,000	1,299,000	0
Shares used for calculating diluted EPS attributable to common stock (in shares) | shares										132,721,000	139,206,000	137,536,000
Earnings Per Share, Basic [Abstract]
Basic Net Earnings (Loss) from Continuing Operations Per Share (in dollars per share) | $ / shares		$ 0.53	$ 1.74	$ (0.92)	$ 1.35	$ 0.16	$ 0.65	$ 0.83	$ 0.45	$ 2.69	$ 2.11	$ (0.03)
Basic Net Earnings from Discontinued Operations Per Share (in dollars per share) | $ / shares		0.10	0.14	0.23	0.58	0.89	3.18	(0.42)	0.42	1.05	4.03	1.21
Basic EPS (in dollars per share) | $ / shares		0.63	1.88	(0.69)	1.93	1.06	3.83	0.41	0.87	3.74	6.14	1.18
Earnings Per Share, Diluted [Abstract]
Diluted Net Earnings (Loss) from Continuing Operations Per Share (in dollars per share) | $ / shares		0.53	1.73	(0.92)	1.33	0.16	0.65	0.82	0.45	2.67	2.09	(0.03)
Diluted Net Earnings from Discontinued Operations Per Share (in dollars per share) | $ / shares		0.09	0.14	0.23	0.58	0.88	3.15	(0.41)	0.41	1.04	4.00	1.21
Diluted EPS (in dollars per share) | $ / shares		0.62	$ 1.87	$ (0.69)	$ 1.91	1.04	$ 3.80	$ 0.41	$ 0.86	3.71	$ 6.08	$ 1.18
Antidilutive securities excluded from computation of ESP (in shares) | shares											1,176
Share Repurchases
Amount authorized to be repurchased	$ 1,000,000,000.0
Accelerated share repurchases, payment	$ 500,000,000												$ 250,000,000	$ 250,000,000
Number of financial institutions | financial_institution											2
Average price per share (in dollars per share) | $ / shares	$ 81.68
Number of shares repurchased (in shares) | shares	4,129,003,000
Common stock, par value (in dollars per share) | $ / shares		1				1				1	$ 1
Preferred stock, par value (in dollars per share) | $ / shares		$ 1				$ 1				$ 1	$ 1
Remaining authorized repurchase amount		$ 57,900,000								$ 57,900,000