Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended													12 Months Ended
		Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[1]	Sep. 30, 2017	[2]	Jun. 30, 2017	[3]	Mar. 31, 2017	[4]	Dec. 31, 2018	[5]	Dec. 31, 2017		Dec. 31, 2016
Earnings Per Share [Abstract]
Net (loss) income attributable to McDermott		$ (2,771)		$ 2	$ 47	$ 35	$ 26		$ 95		$ 36		$ 22		$ (2,687)		$ 179	[6]	$ 34	[6]
Dividends on redeemable preferred stock		(3)	[7]												(3)
Accretion of redeemable preferred stock		(1)	[7]												(1)
Net (loss) income attributable to common stockholders		$ (2,775)		$ 2	$ 47	$ 35									$ (2,691)		$ 179	[6]	$ 34	[6]
Weighted average common stock (basic)															150		91	[6]	80	[6]
Effect of dilutive securities:
Restricted stock and tangible equity units	[6]																4		15
Potential dilutive common stock															150		95	[6]	95	[6]
Net (loss) income per share attributable to common stockholders
Basic:		$ (15.33)		$ 0.01	$ 0.33	$ 0.12	$ 0.28		$ 1.00		$ 0.38		$ 0.27		$ (17.94)		$ 1.97	[6]	$ 0.43	[6]
Diluted:		$ (15.33)		$ 0.01	$ 0.33	$ 0.12	$ 0.27		$ 1.00		$ 0.38		$ 0.23		$ (17.94)		$ 1.88	[6]	$ 0.36	[6]

[1] Operating results for the fourth quarter of 2017 were primarily driven by higher fabrication and marine activity in the MENA segment and installation progress on the Inpex Ichthys project.

[2] Operating results for the third quarter of 2017 reflect high quality operational performance, while operating with peak levels of utilization in the MENA segment and progressing on the Inpex Ichthys project.

[3] Operating results for the second quarter of 2017 continued to reflect efficient project execution and higher engineering, fabrication and marine activity and improved productivity on multiple projects primarily in our MENA segment.

[4] The first quarter of 2017 operating results improvement was driven by strong cost management, with significantly lower project costs in our MENA and APAC segments, and improvements in our selling, general and administrative expenses.

[5] The effects of restricted stock, warrants and redeemable preferred stock were not included in the calculation of diluted earnings per share for 2018 due to the net loss for the period.

[6] Approximately 0.5 million and 0.7 million shares underlying outstanding stock-based awards in 2017 and 2016 were excluded from the computation of diluted earnings per share during those periods because the exercise price of those awards was greater than the average market price of our common stock, and the inclusion of such shares would have been antidilutive in each of those years.

[7] Represents dividends paid on the shares of 12% Redeemable Preferred Stock and accretion of the stock over the seven years from November 29, 2018 through the expected redemption date of November 29, 2025, using the effective interest method.  See Note 21, Redeemable Preferred Stock, for further discussion.