Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 27, 2019	Sep. 28, 2018
KeyW Holding Corporation
Business Acquisition [Line Items]
Revenues	$ 13,068.7
Net earnings	326.0
Net earnings (loss) attributable to Jacobs	$ 303.0
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share)	$ 2.19
Diluted earnings (loss) per share (in dollars per share)	$ 2.17
Business acquisitions pro forma income tax expense (benefit)	$ 41.3
CH2M HILL Companies, Ltd.
Business Acquisition [Line Items]
Revenues		$ 16,012.4
Net earnings		196.3
Net earnings (loss) attributable to Jacobs		$ 184.5
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share)		$ 1.28
Diluted earnings (loss) per share (in dollars per share)		$ 1.27
Business acquisitions pro forma income tax expense (benefit)		$ 409.7