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Other Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - KeyW Holding Corporation $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 27, 2019 USD ($) $ / shares
Business Acquisition [Line Items]
Revenues	$ 13,068.7
Net earnings of the Group	326.0
Net earnings attributable to Jacobs	$ 303.0
Net earnings attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share) | $ / shares	$ 2.19
Diluted earnings (loss) per share (in dollars per share) | $ / shares	$ 2.17

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