Redeemable Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions				3 Months Ended										12 Months Ended				13 Months Ended
		Oct. 21, 2019	Nov. 29, 2018	Dec. 31, 2019		Sep. 30, 2019	[2]	Jun. 30, 2019	[2]	Mar. 31, 2019	[2]	Dec. 31, 2018	[2],[3]	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2019
Temporary Equity [Line Items]
Shares issued	[1]													13,000,000		2,000,000
Preferred stock dividend rate		14.00%
Initial excise price per share		$ 0.01
Payment of directly related issuance costs																$ 18.0
Redeemable preferred stock accreted value		3.00%
Dividends, paid-in-kind		15.00%
Adjusted percentage of outstanding common stock		1.50%
Dividends paid				$ 14.0	[2]	$ 10.0		$ 10.0		$ 10.0		$ 3.0		$ 44.0	[4]	3.0	[4]
Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock dividend rate			12.00%											12.00%
Accreted value of redeemable preferred stock				$ 1,000.00										$ 1,000.00				$ 1,000.00
Preferred stock dividend rate per quarter														3.00%
Preferred stock applicable dividend rate when prior dividend unpaid														13.00%
Period of accretion adjustment to retained earnings deficit														7 years
Expected redemption date														Nov. 29, 2025
Cumulative catch-up adjustment to opening Retained earnings (Accumulated deficit)																		$ 17.0
Redeemable preferred stock				$ 290.0										$ 290.0				290.0
Adjusted accretion and PIK on redeemable preferred stock														44.0
Dividends paid																$ 3.0
Series A Warrants [Member]
Temporary Equity [Line Items]
Fair value of warrants				5.0										$ 5.0				5.0
Consent and waiver agreement entered date														Oct. 21, 2019
Private Placement [Member] | Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued			300,000
Preferred stock dividend rate			12.00%
Preferred stock par value			$ 1.00
Private Placement [Member] | Series A Warrants [Member]
Temporary Equity [Line Items]
Warrants to purchase common shares			6,800,000
Initial excise price per share			$ 0.01
Proceeds from issuance of warrants, net of discount			$ 289.5											$ 289.5
Payment of directly related issuance costs			$ 18.0											18.0
Fair value of warrants				$ 43.0										$ 43.0				$ 43.0

[1] In December 2019, in connection with the Superpriority Credit Agreement, discussed in Note 13, Debt, we issued approximately 11 million shares of McDermott common stock, 0.09 million of Series B Warrants (that will entitle each holder to purchase one share of common stock of McDermott, par value $1.00 per share, at a purchase price of $0.01 per share) (the “Series B Warrants” and, together with the Series A Warrants, the “Warrants”) and 0.56 million shares of Series A Preferred Stock with par value of 0.001 per share, to certain Superpriority Credit Agreement lenders.

[2] 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

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

[4] ^{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.}