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, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2019	[2]	Dec. 31, 2018	[2]	Dec. 31, 2017
Earnings Per Share [Abstract]
Net (loss) income attributable to McDermott		$ (848)		$ (1,873)		$ (132)		$ (56)		$ (2,771)		$ 2		$ 47		$ 35		$ (2,909)		$ (2,687)		$ 179	[3]
Dividends on redeemable preferred stock		(14)	[4]	(10)	[4]	(10)	[4]	(10)	[4]	(3)	[4]							(44)		(3)
Accretion of redeemable preferred stock		(4)	[4]	(4)	[4]	(4)	[4]	(4)	[4]	(1)	[4]							(16)		(1)
Net (loss) income attributable to common stockholders		$ (866)		$ (1,887)		$ (146)		$ (70)		$ (2,775)		$ 2		$ 47		$ 35		$ (2,969)		$ (2,691)		$ 179	[3]
Weighted average common stock (basic)																		182		150		91	[3]
Effect of dilutive securities:
Restricted stock and tangible equity units	[3]																					4
Potential dilutive common stock																		182		150		95	[3]
Net (loss) income per share attributable to common stockholders
Basic:		$ (4.69)		$ (10.37)		$ (0.80)		$ (0.39)		$ (15.33)		$ 0.01		$ 0.33		$ 0.12		$ (16.31)		$ (17.94)		$ 1.97	[3]
Diluted:		$ (4.69)		$ (10.37)		$ (0.80)		$ (0.39)		$ (15.33)		$ 0.01		$ 0.33		$ 0.12		$ (16.31)		$ (17.94)		$ 1.88	[3]

[1] Results in the second, third and fourth quarters of 2018 reflect impacts of the Combination from the Combination Date.

[2] ^{The effects of restricted stock, warrants and redeemable preferred stock were not included in the calculation of diluted earnings per share for 2019 and 2018 due to the net losses for each such year.}

[3] ^{Approximately 0.5 million shares of underlying outstanding stock-based awards in 2017 were excluded from the computation of diluted earnings per share during such year 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 during such year.}

[4] Represents dividends paid and accrued 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