Stockholders' Equity (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 10,388.9	$ 13,367.4	$ 5,002.1
Other comprehensive income (loss), net of tax	(48.5)	(360.6)	98.3
Ending balance	7,688.1	10,388.9	13,367.4
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,234.4)	(1,014.6)
Other comprehensive income (loss) before reclassifications, net of tax	16.3	(178.7)
Reclassification adjustment for net (gains) losses included in net income, net of tax	(12.0)	(41.1)	0.0
Other comprehensive income (loss), net of tax	4.3	(219.8)
Ending balance	(1,230.1)	(1,234.4)	(1,014.6)
Hedging
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(32.9)	27.8
Other comprehensive income (loss) before reclassifications, net of tax	8.9	(58.7)
Reclassification adjustment for net (gains) losses included in net income, net of tax	18.2	(2.0)
Other comprehensive income (loss), net of tax	27.1	(60.7)
Ending balance	(5.8)	(32.9)	27.8
Defined Pension and Other Post-Retirement Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(92.4)	(16.9)
Other comprehensive income (loss) before reclassifications, net of tax	(82.2)	(74.5)
Reclassification adjustment for net (gains) losses included in net income, net of tax	3.0	(1.0)
Other comprehensive income (loss), net of tax	(79.2)	(75.5)
Ending balance	(171.6)	(92.4)	(16.9)
Accumulated Other Comprehensive Loss attributable to TechnipFMC plc
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,359.7)	(1,003.7)
Other comprehensive income (loss) before reclassifications, net of tax	(57.0)	(311.9)
Reclassification adjustment for net (gains) losses included in net income, net of tax	9.2	(44.1)
Other comprehensive income (loss), net of tax	(47.8)	(356.0)
Ending balance	(1,407.5)	(1,359.7)	(1,003.7)
Accumulated Other Comprehensive Loss attributable to Noncontrolling interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(4.0)	0.6
Other comprehensive income (loss) before reclassifications, net of tax	(0.7)	(4.6)
Reclassification adjustment for net (gains) losses included in net income, net of tax	0.0
Other comprehensive income (loss), net of tax	(0.7)	(4.6)
Ending balance	$ (4.7)	$ (4.0)	$ 0.6