Pension and Postretirement Benefits - Summary of Sensitivity Changes in Certain Assumptions in Pension Plans (Detail) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Compensation And Retirement Disclosure [Abstract]
25-basis-point change in discount rate, Effect on Pretax Pension Expense	$ 52
25-basis-point change in discount rate, Effect on Pension Benefit Obligation	$ 53

Defined benefit plan, effect of twenty five basis point change in discount rate on pension benefit obligation.

No definition available.

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Namespace Prefix:	mdr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration