Stockholders' Equity (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 13,409.4	$ 5,044.1	$ 4,956.4
Other comprehensive income (loss), net of tax	(360.6)	98.3	(15.3)
Ending balance	10,430.9	13,409.4	5,044.1
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,014.6)	(927.1)
Other comprehensive income (loss) before reclassifications, net of tax	(178.7)	(87.5)
Reclassification adjustment for net (gains) losses included in net income, net of tax	(41.1)	0.0	0.0
Other comprehensive income (loss), net of tax	(219.8)	(87.5)
Ending balance	(1,234.4)	(1,014.6)	(927.1)
Hedging
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	27.8	(127.1)
Other comprehensive income (loss) before reclassifications, net of tax	(58.7)	53.7
Reclassification adjustment for net (gains) losses included in net income, net of tax	(2.0)	101.2
Other comprehensive income (loss), net of tax	(60.7)	154.9
Ending balance	(32.9)	27.8	(127.1)
Defined Pension and Other Post-Retirement Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(16.9)	(47.4)
Other comprehensive income (loss) before reclassifications, net of tax	(74.5)	43.2
Reclassification adjustment for net (gains) losses included in net income, net of tax	(1.0)	(12.7)
Other comprehensive income (loss), net of tax	(75.5)	30.5
Ending balance	(92.4)	(16.9)	(47.4)
Accumulated Other Comprehensive Loss attributable to TechnipFMC plc
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(1,003.7)	(1,101.6)
Other comprehensive income (loss) before reclassifications, net of tax	(311.9)	9.4
Reclassification adjustment for net (gains) losses included in net income, net of tax	(44.1)	88.5
Other comprehensive income (loss), net of tax	(356.0)	97.9
Ending balance	(1,359.7)	(1,003.7)	(1,101.6)
Accumulated Other Comprehensive Loss attributable to Non-controlling interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	0.6	0.2
Other comprehensive income (loss) before reclassifications, net of tax	(4.6)	0.4
Other comprehensive income (loss), net of tax	(4.6)	0.4
Ending balance	$ (4.0)	$ 0.6	$ 0.2