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Partnerships and Joint Ventures - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) investment	Dec. 31, 2019 USD ($)
Variable interest entity information
Asset impairments	$ 298.0	$ 0.4	$ 90.4
Number of joint ventures divested \| investment		3
COOEC Fluor
Variable interest entity information
Future funding commitments		$ 26.0
COOEC Fluor
Variable interest entity information
Asset impairments		$ 86.0	257.0
Ownership interest (as a percent)		49.00%
Sacyr Fluor \| Discontinued Operations, Disposed of by Sale \| Energy & Chemicals
Variable interest entity information
Ownership interest (as a percent)		50.00%
Gain (loss) on sale of investments		$ (11.0)
Infrastructure Joint Venture \| Discontinued Operations, Disposed of by Sale
Variable interest entity information
Gain (loss) on sale of investments		8.0
Variable Interest Entity, Not Primary Beneficiary \| Accounts and notes receivable, net
Variable interest entity information
Receivables related to work performed		$ 218.0	$ 149.0
Fluor Corporation \| Partnership \| Majority
Variable interest entity information
Entity's interest (as a percent)		50.00%