Earnings Per Share (Reconciliation Of Number Of Shares Used For Basic And Diluted Earnings Per Share ("EPS") Calculation ) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Attributable to Parent	$ (39.3)		$ (3.9)		$ 11.7		$ (3,256.1)		$ (2,414.0)		$ (119.1)		$ 97.0		$ 20.9			$ (2,415.2)	$ (1,921.6)
Weighted average number of shares outstanding																	448,700	448,000	458,000
Total shares and dilutive securities (in shares)																	448,700	448,000	458,000
Basic earnings per share attributable to TechnipFMC plc (usd per share)	$ (0.09)	[1]	$ (0.01)	[1]	$ 0.03	[1]	$ (7.28)	[1]	$ (5.40)	[1]	$ (0.27)	[1]	$ 0.22	[1]	$ 0.05	[1]	$ (7.33)	$ (5.39)
Diluted earnings (loss) per share (usd per share)	$ (0.09)	[1]	$ (0.01)	[1]	$ 0.03	[1]	$ (7.28)	[1]	$ (5.40)	[1]	$ (0.27)	[1]	$ 0.21	[1]	$ 0.05	[1]	$ (7.33)	$ (5.39)	$ (4.20)
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount																	8,300	5,600	3,500
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount																	4,600	4,000	3,500
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount																	1,800	0	0
Performance shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount																	1,900	1,600	0
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive effect of share-based payment arrangements (in shares)																		0	0
Performance shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive effect of share-based payment arrangements (in shares)																			0
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount																	3,800	4,300	2,700

[1] Basic and diluted earnings (loss) per share are computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share information may not equal annual basic and diluted earnings (loss) per share.