Stockholders' Equity (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 7,688.1	$ 10,388.9	$ 13,367.4
Other comprehensive income (loss), net of tax	(214.4)	(48.5)	(360.6)
Ending balance	4,214.3	7,688.1	10,388.9
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,230.1)	(1,234.4)
Other comprehensive income (loss) before reclassifications, net of tax	(171.1)	16.3
Reclassification adjustment for net (gains) losses included in net income, net of tax		(12.0)	(41.1)
Other comprehensive income (loss), net of tax	(171.1)	4.3
Ending balance	(1,401.2)	(1,230.1)	(1,234.4)
Hedging
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(5.8)	(32.9)
Other comprehensive income (loss) before reclassifications, net of tax	26.8	8.9
Reclassification adjustment for net (gains) losses included in net income, net of tax	13.0	18.2
Other comprehensive income (loss), net of tax	39.8	27.1
Ending balance	34.0	(5.8)	(32.9)
Defined Pension  and Other Post-Retirement Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(171.6)	(92.4)
Other comprehensive income (loss) before reclassifications, net of tax	(92.9)	(82.2)
Reclassification adjustment for net (gains) losses included in net income, net of tax	9.2	3.0
Other comprehensive income (loss), net of tax	(83.7)	(79.2)
Ending balance	(255.3)	(171.6)	(92.4)
Accumulated Other Comprehensive  Loss Attributable to TechnipFMC plc
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,407.5)	(1,359.7)
Other comprehensive income (loss) before reclassifications, net of tax	(237.2)	(57.0)
Reclassification adjustment for net (gains) losses included in net income, net of tax	22.2	9.2
Other comprehensive income (loss), net of tax	(215.0)	(47.8)
Ending balance	(1,622.5)	(1,407.5)	(1,359.7)
Accumulated Other Comprehensive  Loss Attributable to Non-Controlling Interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(4.7)	(4.0)
Other comprehensive income (loss) before reclassifications, net of tax	0.6	(0.7)
Other comprehensive income (loss), net of tax	0.6	(0.7)
Ending balance	$ (4.1)	$ (4.7)	$ (4.0)