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Stockholders' Equity (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning balance			$ 13,409.4
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax	$ 41.1		41.1
Other comprehensive income (loss)	(134.0)	$ 50.3	(309.4)	$ 112.3
Ending balance	12,824.3		12,824.3
Foreign Currency Translation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning balance			(1,014.6)
Other comprehensive income (loss) before reclassifications, net of tax			(241.8)
Other comprehensive income (loss)			(282.9)
Ending balance	(1,297.5)		(1,297.5)
Hedging
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning balance			27.8
Other comprehensive income (loss) before reclassifications, net of tax			(21.0)
Reclassification adjustment for net losses (gains) included in net income, net of tax			(7.2)
Other comprehensive income (loss)			(28.2)
Ending balance	(0.4)		(0.4)
Defined Pension and Other Post-Retirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning balance			(16.9)
Other comprehensive income (loss) before reclassifications, net of tax			0.3
Reclassification adjustment for net losses (gains) included in net income, net of tax			0.6
Other comprehensive income (loss)			0.9
Ending balance	(16.0)		(16.0)
Accumulated Other Comprehensive Loss attributable to TechnipFMC plc
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning balance			(1,003.7)
Other comprehensive income (loss) before reclassifications, net of tax			(262.5)
Reclassification adjustment for net losses (gains) included in net income, net of tax			(47.7)
Other comprehensive income (loss)			(310.2)
Ending balance	(1,313.9)		(1,313.9)
Accumulated Other Comprehensive Loss attributable to Non-controlling interest
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning balance			0.6
Other comprehensive income (loss) before reclassifications, net of tax			0.8
Other comprehensive income (loss)			0.8
Ending balance	$ 1.4		$ 1.4